Add API-doc site, format gate, recipe.yaml, bench

[conorbronsdon]

Rolls the quick wins already proven on mojo-xml and mojo-feed onto this repo:

• Canonical formatting: mojo format output committed as its own commit; new pixi run fmt task.
• Format gate in CI: test workflow now fails if sources aren't mojo-format clean.
• API-doc site: docs/render_api.py (copied from mojo-xml) + pixi run docs renders mojo doc JSON to a single-page HTML API reference; docs.yaml workflow builds and deploys it to GitHub Pages on pushes to main. Generated docs/api.json / docs/api.html are gitignored.
• recipe.yaml: rattler-build recipe distributing the pure-Mojo source tree (not a compiled .mojopkg), same shape as mojo-xml's. Not wired to CI.
• bench/: bench/bench_parse.mojo times render_html over the CommonMark spec corpus (652 documents); pixi run bench builds and runs it. Local task only, not in CI.
• Docstring lint: package docstring summary now starts with a capital letter, so mojo doc emits no warnings.

Local verification: pixi run test 49/49 pass; pixi run docs writes docs/api.html; bench runs (~1.2 ms/corpus pass); pixi run fmt && git diff --exit-code clean.

🤖 Generated with Claude Code