docs: migrate site to Zensical, fix broken examples, document all materializers

[jvanbuel]

Summary

Migrates the documentation site from MkDocs Material to Zensical, and fixes a batch of documentation and packaging bugs found while proofreading.

The site keeps mkdocs.yml as its config. Zensical reads it natively, and upstream recommends against hand-writing zensical.toml while the format is still settling.

Site migration

• Swap mkdocs + mkdocs-material for zensical in the docs dependency group, and change CI from mkdocs build --strict to zensical build --strict.
• Move the seven internal design docs from docs/plans/ to plans/, outside docs_dir. Zensical does not implement exclude_docs (zensical/backlog#65), so those files were silently published. The published page set stays at 18.
• Drop the now-dead exclude_docs key, which Zensical ignores.

Documentation bug fixes

These examples did not run before this change:

• Every metric example was broken. Metric is a Pydantic model, so overriding name / description / unit needs a type annotation and executor needs ClassVar. The docs used bare assignments (name = "..."), which raise PydanticUserError at class definition. Now annotated to match the real plugin code.
• Three fluent-chain examples were invalid Python. CheckHub() followed by an indented .with_metrics(...) raises IndentationError on paste. Wrapped in parens.
• The homepage example crashed on macOS/Windows. CheckUp runs providers in a process pool, so scripts need if __name__ == "__main__":. Added, with a one-line note.

Packaging fix

• Export MarkdownMaterializer from the top-level checkup package. It was registered as an entry point and exported from checkup.materializers, but the top-level re-export was missed in feat: Add MarkdownMaterializer #47, so from checkup import MarkdownMaterializer raised ImportError while its four siblings worked.

Content updates

• Document MarkdownMaterializer and SQLAlchemyMaterializer. The docs claimed three materializers; there are five. Both new sections were verified by running them.
• Point repo URLs at datamindedbe/checkup after the rename from incubator-checkup.
• Document uv rather than pip throughout.
• Replace the horseshoe logo and favicon with a mark cropped from the new banner.

Verification

• uv run zensical build --strict passes with no issues; 18 published pages, no plans/ leak.
• uv run pytest: 210 passed, 1 skipped.
• The Markdown and SQLAlchemy materializer output shown in the docs is real output, not inferred.

Note for reviewers

docs/api/core.md contains signature snippets in ```python fences that are not valid Python (e.g. CheckHub(config_path: Path | None = None)). Left as-is, since that is a normal API-reference convention, but happy to change the fences if you prefer.

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