feat(docs): vitepress site for docs/, mermaid rendering, ci build workflow

Implementation of ADR-0022.

* `vitepress`, `vitepress-plugin-mermaid` and `mermaid` added to
  workspace devDependencies. Build settled at ~9 s for the full
  site, sub-second incremental in dev.
* `docs/.vitepress/config.mts`:
  * `srcExclude` drops `docs/README.md` (git/IDE-only index) and
    `docs/decisions/template.md` (authoring scaffold).
  * `rewrites` maps `decisions/README.md` → `decisions/index.md`
    so `/decisions/` resolves to the curated landing while the
    source stays git-friendly.
  * `ignoreDeadLinks` skips cross-repo refs (`../CLAUDE`,
    `../../apps/**`, `../../notes/**`), `localhost:*` dev URLs,
    and the deliberately-excluded `README`/`template` references —
    the source stays usable from a git/IDE reader without
    breaking the site build.
  * Auto-generated sub-sidebar for `/decisions` walks
    `00NN-*.md` so adding an ADR is one file.
  * Mermaid via `withMermaid` with `securityLevel: 'strict'`.
* `docs/index.md` — VitePress Hero landing with four feature
  cards (Architecture, Decisions, Development, Onboarding).
* `package.json` exposes `docs:dev`, `docs:build`, `docs:preview`.
* `docs/development.md` (line ~5): `[setup/](setup/)` → first
  page, since folder-style links don't resolve cleanly under
  `cleanUrls`. Line 330: wrap `${{ github.* }}` in
  `<code v-pre>…</code>` so VitePress's Vue template parser
  doesn't trip on the curly-brace interpolation.
* `.gitea/workflows/docs-site.yml` — builds on every PR (gate
  on parse errors + genuine dead links), uploads the dist as an
  artifact on push to main. The Mermaid plugin regression fence
  greps for `class="mermaid"` / `<svg>` in ADR-0009's rendered
  HTML.
* `.gitignore` excludes `docs/.vitepress/{cache,dist}/`.

Deployment target stays provisional per ADR-0022; the workflow
publishes a `docs-site` artifact for now. The rsync step lands
once the future infrastructure ADR locks the host.

docs/index.md

@@ -0,0 +1,33 @@
+---
+layout: home
+
+hero:
+  name: APF Portal
+  text: Documentation
+  tagline: Architecture decisions, development guide, and onboarding material for APF France Handicap's web portal.
+  actions:
+    - theme: brand
+      text: Read the architecture
+      link: /architecture
+    - theme: alt
+      text: Browse decisions
+      link: /decisions/
+
+features:
+  - title: Architecture
+    details: System context, container diagram, Nx module boundaries, CI/CD pipeline. Cross-cutting Mermaid diagrams that frame the whole platform.
+    link: /architecture
+    linkText: Open architecture
+  - title: Decisions (ADRs)
+    details: Every non-trivial choice — auth flow, sessions, audit trail, downstream-API access, accessibility, performance budgets, admin app — recorded as MADR 4.0.0 ADRs.
+    link: /decisions/
+    linkText: Open decisions
+  - title: Development guide
+    details: Repo layout, daily commands, observability dev-loop (Jaeger, log↔trace correlation), dependency updates, conventional commits.
+    link: /development
+    linkText: Open development guide
+  - title: Onboarding
+    details: WSL terminal setup, Node + pnpm + Docker, Angular + Nx monorepo bootstrap. Everything a new contributor needs to get a working environment.
+    link: /setup/01-wsl-terminal-setup
+    linkText: Start onboarding
+---