docs: point contributors at geolibre-assets for images and demos (#1768)

* docs: point contributors at the geolibre-assets repo for binaries

Screenshots, GIFs, and sample datasets were being committed straight into
docs/assets, which permanently enlarges the clone for everyone since Git
history keeps them forever. Add a Documentation subsection directing
contributors to opengeos/geolibre-assets instead, with the repo-path to
assets.geolibre.app URL mapping, the per-directory conventions, the format
preferences, and the caveat that published paths are effectively permanent.

* Address CodeRabbit review feedback

- Spell out the hosting contract: geolibre-assets is a plain static host (any
  file type), publishes from `main` via GitHub Pages under its single hostname
  assets.geolibre.app, usually within a minute, with GitHub's 100 MB per-file
  limit.
- Note that some existing pages point at other GeoLibre-operated hosts
  (data.geolibre.app, files.opengeos.org); those URLs keep working and need no
  rewriting, but new assets go to geolibre-assets since that is the one
  contributors can open a PR against.
- State explicitly that the published assets.geolibre.app URL is the exception
  to the "link to files outside docs/ with a full GitHub URL" rule above.

* Address Claude review feedback

- Align the asset-path table's pipe columns; all five rows are now the same
  width. No pre-commit hook formats Markdown, so this would not have been
  fixed automatically.
- Soften the closing sentence, which asserted a stricter invariant than the
  repo satisfies: docs/assets/ also holds an orphaned geolibre-app.png that
  nothing references. It now names the icon as the one live file and says an
  unreferenced file there is a leftover, not a precedent.

Author: giswqs

docs/contributing.md

@@ -197,6 +197,56 @@ links and pages left out of the navigation fail the build. Link to other docs
 pages with a relative path (for example `architecture.md`), and link to files
 outside `docs/` with a full GitHub URL.
 
+### Images, demos, and sample data
+
+**Do not commit screenshots, GIFs, videos, or sample datasets to this
+repository.** Binaries are never really deleted from Git history, so every one
+of them permanently enlarges the clone for everyone, including CI.
+
+Put them in [opengeos/geolibre-assets](https://github.com/opengeos/geolibre-assets)
+instead. It is a plain static host, so any file type works. Anything pushed to
+that repository's `main` branch is published by GitHub Pages at the matching
+path under its one hostname, <https://assets.geolibre.app>, usually within a
+minute:
+
+| Repository path        | Published URL                                      |
+| ---------------------- | -------------------------------------------------- |
+| `images/my-panel.webp` | `https://assets.geolibre.app/images/my-panel.webp` |
+| `demos/my-demo.gif`    | `https://assets.geolibre.app/demos/my-demo.gif`    |
+| `data/sample.parquet`  | `https://assets.geolibre.app/data/sample.parquet`  |
+
+Then reference the published URL from your Markdown:
+
+```markdown
+![Raster style panel](https://assets.geolibre.app/images/raster-style-panel.webp)
+```
+
+This is the one exception to the "link to files outside `docs/` with a full
+GitHub URL" rule above: an asset in `geolibre-assets` is referenced by its
+published `assets.geolibre.app` URL, never by a GitHub blob or raw URL.
+
+A few things worth knowing:
+
+- Use `images/` for stills, `demos/` for animations and screen recordings,
+  `data/` for sample datasets, and `styles/` or `fonts/` for map resources.
+- Prefer **WebP** or AVIF for stills, and PMTiles or GeoParquet for data. The
+  screenshots on this site are WebP.
+- Keep individual files under 100 MB, GitHub's hard limit.
+- Published paths are effectively permanent. Renaming or deleting a file breaks
+  every page already pointing at it, so pick the name once.
+- Assets are served with permissive CORS, so the app can fetch them
+  cross-origin.
+
+Some existing pages point at other GeoLibre-operated hosts, mainly
+`data.geolibre.app` and `files.opengeos.org`. Those URLs keep working and do not
+need rewriting; `geolibre-assets` is simply the one you can open a pull request
+against, so send new assets there.
+
+The only images that belong in this repository are the site chrome under
+`docs/assets/` — currently just the icon `mkdocs.yml` uses for the logo and
+favicon. Do not add to that directory; if you find something there that nothing
+references, it is a leftover, not a precedent.
+
 ## Plugins and extensions
 
 GeoLibre supports external plugins loaded from a zip, a local directory, or a