docs: document the Documents feature

[yohamta0]

Summary

• add a dedicated Documents guide with the file tree, Markdown and Mermaid preview, tabs, drafts, search, permissions, and workspace behavior
• document paths.docs_dir, DAGU_DOCS_DIR, ${context.paths.docs_dir}, and DAG_DOCS_DIR
• cover document ownership across workspace lifecycle operations and Git Sync doc items
• add Documents to the Web UI overview, homepage, navigation, CLI examples, REST API reference, and deployment guidance

Context

Companion documentation for dagucloud/dagu#2491 and dagucloud/dagu#2368.

Validation

• pnpm build
• pnpm test:worker
• pnpm validate:examples

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Summary by cubic

Documents feature is now fully documented: a Web UI for Markdown runbooks with Mermaid, tabs, drafts, search, workspace scoping, and Git Sync. Adds storage configuration (paths.docs_dir/DAGU_DOCS_DIR), runtime paths (${context.paths.docs_dir}/DAG_DOCS_DIR), and REST/CLI references.

• New Features

  • New Documents guide: file tree, tabs, Markdown/Mermaid preview, outline, drafts, search, workspace scope.
  • Web UI overview updated and sidebar link added; homepage cards highlight Documents.
  • Server config: paths.docs_dir and DAGU_DOCS_DIR, defaults, layout, and workspace storage rules.
  • Runtime: ${context.paths.docs_dir} and DAG_DOCS_DIR added to context/env docs.
  • REST API: Documents endpoints for list, search, read, create, update, rename/move, delete, batch delete.
  • Git Sync: documents tracked as doc items under docs/; CLI examples for publish/delete/mv.
  • Workspaces: document ownership, rename moves tree, deletion blocked while documents exist; search updated.
  • Redirects: /features/documents now points to /web-ui/documents.

• Migration

  • Set paths.docs_dir (or rely on {dags_dir}/docs); optionally set DAGU_DOCS_DIR.
  • Add docs/ to your Git repo if using Git Sync; documents appear as doc items.
  • Review workspace policies: create/modify docs in a specific workspace; cannot delete a workspace until its docs are removed.

^{Written for commit dd489e1. Summary will update on new commits.}