ci: add markdown lint workflow for PR validation

[allexistence]

Summary

Add a GitHub Actions workflow to lint Markdown files on pull requests.

Changes

Add markdown lint workflow under .github/workflows
Runs on pull requests for .md files only

Why

Helps catch formatting issues early and improves documentation consistency.

[stmcginnis]

This is likely to be a bit contentious. Since the markdown files in this repo have been around for years with no linting happening, I know there are going to be many errors flagged by markdownlint. That will create a lot of churn for limited value.

That said, I do see value in validating markdown format. Other repos where I've used, it has helped find some subtle formatting issues. So if we're going to accept something like this, we will probably need to start off disabling most of the checks, then enable incrementally and address issues in smaller chunks. Otherwise the diff is just going to be too large to be able to do any meaninful review of the changes.

[BenTheElder]

We usually use lint tools that can be replicated locally via a make target, we'd want something like that versus depending on some external action definition.

We could add ratcheting validation.

That said, I'd want to look at what e.g. kubernetes/website uses, since we have more mature documentation projects that generally establish best practices for the organization.

We do already run a spellcheck tool, make sure the markdown builds to a site, and view the preview of that site.

We don't rely heavily on e.g. frontmatter, and our markdown isn't pure markdown since it has hugo shortcodes ...