Add MkDocs documentation site, internals section, and rewritten README

[Korijn]

Track B of the roadmap in a single PR (docs site + internals + README rewrite), following the approach of observ #176, #179 and #180.

Docs site

• MkDocs + Material theme (mkdocs.yml, new docs dependency group), plus mkdocstrings for the generated API reference — same config shape as observ's.
• GitHub Actions workflow (.github/workflows/docs.yml) that builds the site with uv and deploys it to GitHub Pages on push to master.
• Content:
  • Getting Started: installation, quick start (diff → apply → serialize → produce).
  • Guide: diffing semantics, applying patches (apply vs iapply, ordering, undo/redo), JSON pointers, produce() and drafts (including in_place and live-path tracking), serialization and its non-JSON caveats, observ integration, gotchas & best practices (including the deliberate RFC 6902 divergences: sets, tuples, non-string tokens, no move/copy/test).
  • Reference: API reference generated from docstrings.
  • Internals: architecture page covering the DP list diff with prefix/suffix trimming and traceback padding, Pointer.evaluate's strict-parent/tolerant-leaf resolution, and produce()'s proxy design (parent-link paths, detachment/adoption semantics, record-time snapshotting, the reader-method generation).
• Docstrings added to diff, apply, iapply, to_json, to_str_paths and Pointer (no behavioral changes) so the API reference has content, with cross-links between them.

README rewrite

Links up front (docs site, quick start, API reference), sharper pitch, slimmed to pitch + two quick examples pointing at the docs site, and an observ referral for the reactive-state use case.

Verification

• All 34 Python code blocks across the docs and README are executed and verified against the current implementation (extraction script runs every block; printed JSON output blocks match real output).
• The site builds without warnings using the exact CI recipe (uv sync --only-group docs + uv run --no-sync mkdocs build), including all mkdocstrings cross-references.
• Full test suite passes at 100% coverage; ruff check and format pass.

Note: before the first deploy, GitHub Pages needs to be set to "GitHub Actions" as the source in the repository settings.

🤖 Generated with Claude Code

https://claude.ai/code/session_016Mu9vEBwU4fQLi8ZgkgdS2

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