Add lint against linking into main branch on GitHub

[emilk]

What

While investigating recently broken links (because of #5966 (comment)) I decided to add a lint against linking to stuff on changing branches (e.g. /blobs/main).

Unfortunately I cannot cover it all (see #6077 for more), but this is an OK start.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

To run all checks from main, comment on the PR with @rerun-bot full-check.

[github-actions]

Deployed docs

Commit	Link
9e181ac	https://landing-p434j5szz-rerun.vercel.app/docs

[Wumpf]

lgtm!