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

[conorbronsdon]

Ports the quality-of-life setup already proven on mojo-xml and mojo-feed to this repo:

• Format-clean baseline: one-time mojo format commit over src/, test/, examples/, plus a CI format gate in test.yml (mojo format + git diff --exit-code).
• API-doc site: docs/render_api.py (copied verbatim from mojo-xml) renders mojo doc JSON into a self-contained HTML reference; .github/workflows/docs.yaml builds and deploys it to GitHub Pages. Generated docs/api.json / docs/api.html are gitignored.
• pixi tasks: fmt, docs, and bench added alongside the existing test / demo.
• recipe.yaml: rattler-build recipe distributing the pure-Mojo source tree (same rationale and shape as mojo-xml's recipe), not wired to CI.
• bench/: small bench_parse.mojo timing parse_captions over the bundled SRT/VTT fixtures; local pixi run bench only, not in CI.
• Capitalized the package docstring summary in src/captions/__init__.mojo.

Verified locally: 32/32 tests pass, pixi run docs renders docs/api.html, bench compiles and runs, and pixi run fmt && git diff --exit-code is clean.

🤖 Generated with Claude Code