Rework README.md and extract contribution guidelines to CONTRIBUTING.md

AGENTS.md

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+# AGENTS.md
+
+This repository contains the Scalingo Documentation Center.
+
+Before making changes, read:
+
+- `README.md` for repository context and structure
+- `CONTRIBUTING.md` for contribution rules, content conventions, redirects, and validation steps
+
+Agent-specific guidance:
+
+- Keep edits minimal and scoped to the user request.
+- Do not duplicate rules from `README.md` or `CONTRIBUTING.md` here.
+- Prefer updating canonical documentation rather than adding new local instructions.
+- When moving or renaming documentation pages, update internal links and add the corresponding redirect in redirections.yml in the same change.
+- If a repository rule conflicts with an explicit user request, follow the user request.

CONTRIBUTING.md

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+# How to contribute
+
+[*Scalingo Documentation Center*](http://doc.scalingo.com) relies on Jekyll. You are welcome to contribute to this documentation by forking and sending pull requests.
+
+## Repository structure
+
+Jekyll reads its source files from `src/` because `_config.yml` sets `source: src`.
+
+Main content is organized in:
+
+- `src/_posts/` for documentation pages
+- `src/_tutorials/` for tutorials
+- `src/_tutorials_categories/` for tutorial categories
+- `src/changelog/` for changelog entries
+- `src/_includes/` for reusable Liquid partials
+- `assets/` for source assets
+- `src/assets/` for built assets
+
+## Adding a new article
+
+To add a new content page, create a new markdown file in the appropriate directory for that content type, using the following file format: `yyyy-mm-dd-title.md` (*e.g. 1970-01-01-what-is-epoch.md*).
+Please use the current date when creating the file.
+The date in the filename must be less than or equal to the current build date, otherwise the page may be excluded from the build output.
+
+The minimal *front matter* that you need to add is:
+
+```yml
+---
+title: What is Epoch
+modified_at: 1970-01-01 00:00:00
+---
+```
+
+Optional tags:
+
+- `nav: [string]` set another url than the one generated from the title
+- `index: [integer]` change the position of the page in his folder in the side menu
+- `tags: [string]` add search and classification keywords
+- `layout: [string]` use a specific page layout for special pages
+- `description: [string]` add a short page description used by some landing pages
+- `subnav_index: [integer]` set the position of a page in the sub-navigation
+- `logo: [string]` define the icon or logo used by tutorial pages
+- `products: [array]` list related Scalingo products for a tutorial
+- `category: [string]` assign a tutorial to a tutorial category
+- `permalink: [string]` define the public URL of tutorials and other special pages that require a stable path
+- `is_series: [boolean]` mark a tutorial as part of a series
+- `series: [string]` group tutorial pages under the same series name
+- `series_index: [integer]` set the position of a page inside a tutorial series
+- `oses: [array]` list supported operating systems on pages that display OS-specific icons
+- `cvss: [object]` define CVSS metadata for security bulletins
+- `github: [string]` link a changelog entry to the upstream GitHub repository
+
+The exact set of supported tags depends on the content type and layout. When in doubt, copy the front matter structure from a nearby page of the same kind.
+
+## Modifying an article
+
+You are welcome to modify any article, but please remember to update `modified_at` before sending your pull request. In existing content, `modified_at` is typically set to `00:00:00`; prefer keeping that convention unless you have a reason to do otherwise.
+
+When adding or editing content, prefer following the existing structure and naming conventions used by nearby files.
+
+## Content conventions
+
+### Don'ts
+
+Please do not use the `date` metadata as it will conflict with the date extracted from the file name. Instead, use `modified_at` to record when a modification is made to an article.
+
+Please do not use first-level HTML/Markdown headers (*i.e. `<h1></h1>`*) as it will be pulled from the `title ` metadata.
+
+__Blockquotes__ should be **only used for quotes**.
+
+Do not put `categories` in the *front matter*. Avoid adding `category` or `permalink` to regular documentation pages in `src/_posts/` unless the content type already relies on them, such as tutorials or some legacy pages.
+
+Do not manually edit generated files in `src/assets/` unless the task explicitly requires it. Prefer editing the source files in `assets/`.
+
+### Do's
+
+If you want to write a useful note:
+
+```liquid
+{% note %}
+  My useful note here
+{% endnote %}
+```
+
+If you want to write a warning note:
+
+```liquid
+{% warning %}
+  My warning note here
+{% endwarning %}
+```
+
+If you want to insert a link to another documentation article:
+
+```markdown
+[text of the link]({% post_url platform/internals/2000-01-01-routing %})
+```
+
+### Links
+
+Prefer reference-style Markdown links when a page contains several links or when the target URL is long.
+
+Example in content:
+
+```markdown
+Open your [account dashboard][dashboard].
+Transfer ownership with [this guide][transfer-project-ownership].
+```
+
+Then define the links at the bottom of the page:
+
+```markdown
+[dashboard]: https://dashboard.scalingo.com/
+[transfer-project-ownership]: {% post_url platform/projects/2000-01-01-manage-projects %}#transfer-project-ownership
+```
+
+This keeps the body of the page easier to read and makes link maintenance simpler.
+
+For short, isolated links, inline Markdown links are still acceptable.
+
+To insert an image, first upload it to our CDN, inside the documentation
+folder. Give it a public permission `Grant public read access to this
+object(s)`. Then, insert it with:
+
+```liquid
+{% assign img_url = "https://cdn.scalingo.com/documentation/screenshot_*" %}
+{% include mdl_img.html %}
+```
+
+## Renaming a page
+
+If you rename a page or change its path, check the root-level files that may contain hardcoded internal URLs or routing rules related to that page, especially:
+
+- `_config.yml`
+- `config.ru`
+- `redirections.yml`
+
+If the public URL changes, add a new redirect entry in the `301` section of `redirections.yml` in the same change.
+
+New redirect entries must be inserted above the `obsolete` section. Existing redirect entries should not be rewritten, reordered or removed as part of a normal page move.
+
+Example:
+
+```yml
+"301":
+  -
+    old: "/old/path"
+    new: "/new/path"
+```
+
+## Running Locally
+
+To install dependencies locally:
+
+```sh
+docker compose build --pull \
+&& docker compose run --rm web bundle install \
+&& docker compose run --rm web yarn install --ignore-engines \
+&& docker compose run --rm web bundle exec jekyll build
+```
+
+To build the static site and spin-up a file server:
+
+```sh
+docker compose up
+```
+
+And visit http://localhost:4300
+
+If you want to serve the doc like in production (through the rack stack), generate the site first (see above) and then:
+
+```sh
+docker compose -f docker compose-prod.yml up
+```
+
+This will run puma in parallel and serve the site at http://localhost:4302
+
+### Manual rebuilds for changelog, samples, and new sections
+
+For a reason I ignore and I don't want to spend time understanding, we need to manually re-build the pages when adding a new changelog entry or a new section.
+This is done with:
+
+```sh
+docker compose exec web bundle exec jekyll build
+```
+
+### How to debug
+
+#### Jekyll build
+
+Using [debug.rb](https://github.com/ruby/debug) it's possible to add break points in Ruby code.
+1. Prepend command `jekyll serve` by `rdbg -c -- `
+2. Add a `binding.break` where you want to stop the execution
+
+#### Theming (HTML/CSS)
+
+To help debug responsive layout issues add this tool in the default layout, it will show the current Tailwind screen & size in every pages.
+
+```liquid
+  {% include organisms/responsive_tool.html %}
+```
+
+#### Force regeneration
+
+Sometimes, the files regeneration (especially the assets) got lost.
+
+In this case remove the `_site` folder via
+```sh
+docker compose run web rm -rf _site
+```
+
+## Validation
+
+After content or configuration changes, run:
+
+```sh
+docker compose run --rm web bundle exec jekyll build
+```
+
+After asset changes, run:
+
+```sh
+docker compose run --rm web yarn build
+docker compose run --rm web bundle exec jekyll build
+```