✅ Add link checker

[CollierCZ]

Why

We want to ensure all our links are still valid.
Closes #2049

What's changed

• Added muffet as a link checker to crawl and check links.
  • It needs to be rate limited for sites like GitHub, which means it takes a while.
  • It's also flaky, especially with Community sometimes timing out for no reason.
• So moved it to a twice-weekly check that will update 🐛 Broken links #2213 with any broken links found.
• Added an internal link checker to test each PR that's submitted.
  • Vault is excluded because it uses headings from transclusion (so they're marked as missing). If there's a problem, it should be caught in the regular check.
• Fixed links identified by both checks.
• Added checks to CONTRIBUTING.md

[trolologuy]

Great work! That's quite a massive PR. I can't wait to see the automatic link checker live!
A few proposed changes here and there, but nothing big either.

• Final built pages are working as expected.
• The information is correct.
• The information is comprehensive.
• The information is concise.
• The information is clear.
• All links work, including to section headers.

✅ Okey to merge.

[trolologuy]

Thanks for the changes, looks better now 🎉