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

[conorbronsdon]

Rolls the quick-win templates from mojo-xml / mojo-feed onto this repo:

• Canonical format commit: mojo format output committed as its own commit, so the new CI gate starts green.
• Format gate in CI: test workflow now fails if sources are not mojo format clean (run pixi run fmt to fix).
• API-doc site: docs/render_api.py + docs.yaml workflow render mojo doc JSON to a static HTML API reference (Pages deploy; enabling Pages is a separate step). New pixi run docs task builds docs/api.html locally.
• recipe.yaml: rattler-build recipe documenting the intended conda distribution shape (source tree + runtime mojo pin, not a compiled .mojopkg). Not wired to CI.
• bench/bench_diff.mojo + pixi run bench: unified_diff timing over the repo's own test fixtures. Fixtures are tiny, so each pair runs 20k iterations for stable numbers; local-only, not in CI.
• New fmt / docs / bench pixi tasks; .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: all 34 tests pass, pixi run docs builds docs/api.html, bench compiles and runs, and pixi run fmt && git diff --exit-code passes.

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