ci: add link checker for documentation URLs

[adamdaw]

Add a CI step that validates external URLs in documentation files don't return 404.

Context

README.md, CLAUDE.md, CONTRIBUTING.md, and docs/ contain 50+ external URLs to standards bodies, tool documentation, and references. These can break over time (as demonstrated by the iso25010.info outage).

Acceptance Criteria

• Link checker runs in CI (lychee or broken-link-checker)
• Covers *.md files in repo root and docs/
• Allows configurable exclusions for known-flaky URLs
• Fails build on 404s, warns on timeouts

Reference: github.com/lycheeverse/lychee