📝 docs: add a Zensical site deployed to GitHub Pages

[nstarman]

Adds a Zensical docs site, published to GitHub Pages at https://galacticdynamics.github.io/xmmutablemap. The setup follows nstarman/quax.

Changes

• mkdocs.yml: Material theme with a light/dark toggle, using the repo's favicon.svg as the logo.
  • Home page: docs/index.md pulls in README.md with pymdownx.snippets, so the README remains the only source. github-callouts (from markdown-callouts) renders the README's > [!WARNING] alert as a warning admonition.
  • API page: docs/api.md documents ImmutableMap and frozendict with mkdocstrings, using docstring_style: numpy to match the docstrings.
• pyproject.toml: a new docs dependency group, also included in dev, with uv.lock updated.
• .github/workflows/docs.yml: builds on every PR and on workflow_dispatch. On main it uploads the Pages artifact and deploys with actions/deploy-pages. The concurrency group matches ci.yml (👷 ci: key main's concurrency group on push events only #78), so deploys from separate merges don't cancel each other. quax's workflow builds twice; this one builds once and uploads conditionally.

Pages is already set to deploy from GitHub Actions in the repo settings, so the first merge to main should publish the site.

Verification

• In a clean environment, uv sync --no-default-groups --group docs --locked && zensical build --clean reports no issues.
• I checked the rendered home and API pages locally. The README warning renders as an admonition, the anchor links work, and the NumPy-style docstrings render as parameter tables with formatted examples.
• prek run --all-files passes and pytest passes (116 tests).

To preview locally: uv run --group docs zensical serve

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@6f98352). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff            @@
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  Coverage        ?   100.00%           
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  Files           ?         3           
  Lines           ?        75           
  Branches        ?         0           
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  Hits            ?        75           
  Misses          ?         0           
  Partials        ?         0           

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