[docs] Add auto-generated API reference with mkdocstrings

[dgenio]

Problem

contextweaver has no API reference documentation. Users must read source code to discover the API surface. This is a critical DX gap. The codebase already has comprehensive Google-style docstrings — the raw material exists, it just is not rendered.

Proposal

Add a documentation site using MkDocs + mkdocstrings that auto-generates API reference pages from existing docstrings.

1. Add mkdocs.yml configuration with Material for MkDocs theme
2. Add docs/reference/ directory with per-module pages (types, config, protocols, envelope, context/manager, routing/catalog, routing/router, adapters/mcp, adapters/a2a, store modules)
3. Add [project.optional-dependencies] docs = [...] extra in pyproject.toml
4. Add make docs target to build locally
5. Optionally add a GitHub Pages deployment workflow

Acceptance Criteria

• mkdocs.yml configured with mkdocstrings and Material theme
• API reference pages generated for all public modules
• pip install -e ".[docs]" installs documentation build tools
• make docs builds the docs site locally
• Existing docs (architecture.md, concepts.md) integrated into the site
• README updated with link to docs site

File Paths

• mkdocs.yml (new)
• docs/reference/ (new)
• pyproject.toml (edit)
• Makefile (edit)

References

• mkdocstrings: https://mkdocstrings.github.io/
• Material for MkDocs: https://squidfunk.github.io/mkdocs-material/