restructure handbook to flat-page spec

Replace the four-section dir layout with the revised flat-page spec:
- Three-page nav: Libraries, Standards, Deploy.
- libraries.md: live client-side fetch of the rethink-public org from the
  Gitea API, denylist-filtered, sorted table linking each repo, no version
  pins, graceful fallback on fetch/CORS failure.
- standards.md: house coding standards.
- deploy.md: deploy guide for rethink-net (services uid 1337, uid-agnostic
  Docker, paths/mounts, bind-mount permissions, read-only secrets).
- index.md landing card grid updated for the three sections.
- gitignore Playwright MCP artifacts.

Verified: mkdocs build --strict clean; libraries fallback renders live.
Signed-off-by: disqualifier <dev@disqualifier.me>

.gitignore

@@ -9,3 +9,6 @@ __pycache__/
 *.py[cod]
 .venv/
 venv/
+
+# Playwright MCP run artifacts
+.playwright-mcp/