feat: CI link validation for MDX content

[moss-bryophyta]

Summary

Adds a TypeScript-based link validator that runs in CI on every PR.

What it validates

• Internal links (starting with /) resolve to actual files
• Anchor fragments (#heading) exist in the target file
• Same-page anchors validated against current file headings
• Custom heading IDs via [#id] syntax (e.g. ## My Heading [#custom-id])
• Locale-optional paths — /docs/next/intro and /en-US/docs/next/intro both valid
• Template placeholder links — expands __DOCS_PATH__ etc. for next/react/react-native and validates each

Files

• scripts/validate-links.ts — main validator (mdast AST parser + github-slugger)
• scripts/test-validate-links.ts — unit tests
• scripts/package.json + lockfile + .gitignore
• .github/workflows/validate.yml — added validate-links job

Note

There are 33 pre-existing broken links in the current content (missing files in react-native, empty generated pages, stale anchors). These are real issues, not false positives. CI will flag them on this PR — they can be fixed in a follow-up or in this PR before merge.