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.

docs/mkdocs.yml

@@ -1,5 +1,4 @@
 site_name: Mealie
-demo_url: https://demo.mealie.io
 site_url: https://docs.mealie.io
 use_directory_urls: true
 theme:
@@ -16,7 +15,7 @@ theme:
       toggle:
         icon: material/weather-sunny
         name: Switch to light mode
-  custom_dir: docs/overrides
+  custom_dir: overrides
   features:
     - content.code.annotate
     - content.code.copy
@@ -50,6 +49,8 @@ markdown_extensions:
           class: mermaid
           format: !!python/name:pymdownx.superfences.fence_code_format
   - pymdownx.details
+extra:
+  demo_url: https://demo.mealie.io
 extra_css:
   - assets/stylesheets/custom.css
 extra_javascript: