feat: emit per-file annotations for broken links in CI build

[jhutchings1]

Summary

@kodster28: I seem to keep getting link validation issues when I'm using Opencode to write new content, and I found that scrolling through the log was kind of cumbersome. Open to a change like this that would create per line annotations of the link validation issues in the source?

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/.github/	@cloudflare/content-engineering, @kodster28, @mvvmm, @colbywhite, @ahaywood, @MohamedH1998
*.ts	@cloudflare/content-engineering, @kodster28

[kodster28]

Can you elaborate on what the expected output would be, in a few senses:

• Screenshot / demo of the behavior
• Time for this action to run
• Whether blocking CI or non-blocking

For most folks, we've found that running this check locally works pretty well. Run a build with RUN_LINK_CHECK to true as a local env. ref

[github-actions]

Preview URL: https://623ee94f.preview.developers.cloudflare.com
Preview Branch URL: https://jhutchings1-link-validation-improvements.preview.developers.cloudflare.com