docs: generate API reference for src/mlx_benchmarks

[JacobPEvans-personal]

Why

Today src/mlx_benchmarks/ (envelope, publish, converters, system) has Python docstrings but no rendered API reference. Users have to read __init__.py to discover the public surface (e.g., get_converter(), ConverterContext, detect_system()).

Suggested approach

• Generate docs with pdoc or sphinx-autodoc (whichever is smaller).
• Publish to GitHub Pages or fold into docs/ so it links from README.
• Cover the public surface only: mlx_benchmarks.envelope, mlx_benchmarks.publish, mlx_benchmarks.converters, mlx_benchmarks.system. Skip cli and logging_config.
• Add a docs: workflow that runs on main and surfaces the build.

Acceptance

• README links to a hosted API reference.
• The reference shows class/function signatures with docstrings rendered.
• mlx-bench-publish and get_converter() are findable without grepping source.

Filed as follow-up to the docs(presentation): polish work in #49.