Documentation site (mkdocs) instead of plain markdown in docs/

## Context

Documentation currently lives as flat markdown under `docs/` (`concepts.md`, `usage.md`, …) and is linked from the README. A built site would give navigation, search, and an API reference generated from docstrings.

## Proposal

- `mkdocs` + `mkdocs-material` + `mkdocstrings[python]` for the API reference.
- Existing `docs/*.md` become the site's pages with minimal changes.
- Deploy to GitHub Pages from a workflow on push to `main`.
- Add a "Docs" badge/link to the README.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start by reviewing the existing docs/*.md pages and their links in README. Then map the proposed MkDocs site, Python API reference, and GitHub Pages workflow; done means the pages are navigable and searchable, the API reference is generated, deployment runs on pushes to main, and README has a Docs link or badge.

Written by the indexing model from the issue text.