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.

pyproject.toml

@@ -61,12 +61,12 @@ pgsql = [
 
 [dependency-groups]
 docs = [
-    "mkdocs-material==9.7.6",
+    "zensical==0.0.42",
 ]
 dev = [
     "coverage==7.14.1",
     "coveragepy-lcov==0.1.2",
-    "mkdocs-material==9.7.6",
+    "zensical==0.0.42",
     "mypy==2.1.0",
     "pre-commit==4.6.0",
     "pylint==4.0.5",