Check the docs site's internal links in the build #82

The docs repeat a lot of facts about the repositories, and links between pages go stale quietly. starlight-links-validator (free, MIT) fails the build on a broken internal link. External links aren't checked.

Phase 1: The check

• Add starlight-links-validator to the site and pin its version
• Fix whatever it finds
• Mention it in the site README

Checks: npm run build passes, and fails when a link to a missing page is added.