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

[conorbronsdon]

Suite-wide quick wins, following the pattern established in mojo-xml and mojo-feed:

• Canonical format commit + CI format gate: sources committed as mojo format output, and the test workflow now fails if a PR isn't format-clean (pixi run fmt to fix).
• API-doc site: pixi run docs renders mojo doc JSON to a single-page HTML reference via docs/render_api.py; .github/workflows/docs.yaml builds and deploys it to GitHub Pages on push to main.
• recipe.yaml: rattler-build recipe distributing the pure-Mojo source tree with a runtime mojo pin (not a compiler-locked .mojopkg). Not wired to CI.
• bench/bench_render.mojo: parse+render throughput benchmark over the Jinja2 parity corpus (test/data/fixtures/manifest.txt); fixtures are tiny so it runs 2000 corpus passes for stable numbers. Local pixi run bench only, not in CI.
• New pixi tasks: fmt, docs, bench. .gitignore covers the bench binary and generated docs.
• Docstring lint: package summary now starts with a capitalized sentence (mojo doc emits no summary warning).

Local verification: 78/78 tests pass, pixi run docs renders api.html, bench compiles and runs (~4.3 us/render), pixi run fmt && git diff --exit-code clean.

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