ci: Add periodic dead link detection for MkDocs documentation

[Copilot]

No automated link checking existed for the docs/ directory, allowing dead links to accumulate undetected.

Changes

• New workflow .github/workflows/dead_link_check.yml using byrnereese/github-action-mkdocs-link-check@v1
  • Runs on a weekly cron schedule (Mondays 09:00 UTC)
  • Supports workflow_dispatch for on-demand execution
  • Recursively scans docs/ with verbose HTTP status output
  • Scoped to permissions: contents: read (least-privilege GITHUB_TOKEN)

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