docs: document linkcheck caveats for release readiness

[rogerchappel]

Summary

• add README limitations for opt-in external URL checks, lightweight parsing scope, and generated-anchor edge cases
• add a Next changelog entry for the public caveats documentation

Verification

• npm ci (installed worktree dependencies; npm reported 5 existing audit findings)
• npm run release:check
  • npm run check
  • npm test (7 files, 58 tests passing)
  • npm run build
  • npm run smoke (7 smoke checks passing)
  • npm run package:smoke

Risk Level

Low. Documentation and changelog only; no runtime behavior changes.

Rollback Plan

Revert the two documentation commits on this branch.