[docs] Add CI dead link gate

[dataroaring]

Summary

• make the docs changed-link check fail CI on blocking internal link errors
• expand the docs governance manifest to cover current docs, zh-CN current docs, and site page routes such as /download
• fix repo-internal missing targets and anchors across active, archived, localized, and ja-source docs
• improve link linting for compatibility anchors, dotted relative paths, fenced code masking, and report-only severity handling

Validation

• node scripts/docs-governance/lint-links.js --output /tmp/doris-links-all-no404-audit.json
  • link-missing-target = 0
  • link-missing-anchor = 0
  • errors = 0
• yarn docs:links:changed --fail-on-errors --output /tmp/doris-links-changed-no404-audit.json
  • internal_missing = 0
  • errors = 0
• yarn docs-governance:test
• git diff --check

Notes

External links remain report-only and are not fetched by this gate; this PR eliminates repo-internal 404 references covered by the docs link linter.

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