• Added lycheeverse as a external link checker.
• It runs every time that a markdown file is changed.

Here’s why I used these parameters (.github/workflows/links-check.yml):

1. --accept 200,403
   We accept 403 because sites like Medium, npmjs, and Twitter/X block bots/scrapers and commonly return a 403 even when the link is valid. Since these cases are rare in .md files, accepting 403 avoids false positives without affecting the usefulness of the check.

2. --max-concurrency 5
   Limiting concurrency is necessary to avoid services like GitHub returning HTTP 429 (Too Many Requests) for pages that are otherwise accessible. Using 5 threads is enough, it prevents hitting rate limits while still keeping the link check reasonably fast.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct