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

[conorbronsdon]

Ports the quick wins already proven on mojo-xml and mojo-feed:

• Canonical formatting: one-time mojo format commit over src/test (2 files), plus a CI Format check step in test.yml right after Mojo install, so drift fails fast.
• API-doc site: docs/render_api.py (copied verbatim from mojo-xml) + .github/workflows/docs.yaml render mojo doc JSON for src/redis into a self-contained HTML reference; pixi run docs builds it locally. Also adds a module docstring to src/redis/__init__.mojo — mojo doc now emits no summary warnings.
• recipe.yaml: rattler-build recipe (mojo-xml pattern) distributing the pure source tree with this repo's own mojo run pin — documents the intended conda distribution shape, not wired to CI.
• bench/bench_resp.mojo: offline RESP2 benchmark — serializes commands via encode_command and parses canned replies via parse_reply, the same network-free path test/test_resp.mojo exercises, so no Redis server is needed. Local pixi run bench only, not in CI.
• New pixi tasks fmt, docs, bench; .gitignore entries for generated docs and the bench binary.

Verified locally (WSL): protocol tests 36/36, connection tests 2/2, integration tests 21/21 against a live redis-server on port 6399, pixi run docs renders, bench compiles and runs, and pixi run fmt && git diff --exit-code passes. CI's live-test posture is unchanged — the integration suite stays local-only.

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