chore(docs): migrate documentation from MkDocs to Zensical

Material for MkDocs is in maintenance mode; Zensical is its successor from
the same maintainers. Zensical reads the existing mkdocs.yml natively, so
this is a tooling swap rather than a config rewrite.

- pyproject.toml: replace mkdocs-material with zensical in docs/dev groups
- Taskfile/CI/netlify: swap `mkdocs build|serve` for `zensical build|serve`
- mkdocs.yml: move custom `demo_url` under `extra:` (Zensical doesn't expose
  custom top-level config keys to templates); move `custom_dir` out of the
  docs source tree so raw templates aren't published as static files
- home.html: use `config.extra.demo_url`; replace the non-functional
  `lang.t('source.link.title')` (a Python call MiniJinja can't run, already
  rendering as literal text) with a static tooltip

Verified: `zensical build` exits clean ("No issues found"), 46 pages, with
mermaid, admonitions, content tabs, task lists, nav tabs, code-copy, custom
homepage, and announce banner all rendering at parity with the prior build.

.github/copilot-instructions.md

@@ -63,7 +63,7 @@ task setup              # Install all dependencies (Python + Node)
 task dev:services       # Start Postgres & Mailpit containers
 task py                 # Start FastAPI backend (port 9000)
 task ui                 # Start Nuxt frontend (port 3000)
-task docs               # Start MkDocs documentation server
+task docs               # Start Zensical documentation server
 ```
 
 **Code generation (REQUIRED after schema changes):**

.github/workflows/docs.yml

@@ -30,7 +30,7 @@ jobs:
         run: uv sync --only-group docs --no-install-project
 
       - name: Build docs
-        run: uv run --no-project mkdocs build -d site
+        run: uv run --no-project zensical build
         working-directory: docs
 
       - name: Upload artifact