📝 docs: generate CLI reference from the argparse parser

[gaborbernat]

The command line was documented only by a hand-written how-to whose per-flag details drifted from the argparse parser in turbohtml.__main__. 📝 This PR generates an exhaustive CLI reference straight from _parser() with sphinx-argparse-cli, so the documented subcommands and flags track the code the same way the doctest-run examples keep the rest of the docs honest.

The new docs/reference/cli.rst renders the parser under a Command line toctree group; sphinx-argparse-cli walks every subcommand (minify, minify-css, minify-js, detect, to-markdown, to-text, sanitize) and their options. docs/how-to/cli.rst stays the task-oriented guide with the subcommand-to-API mapping and pipeline examples, and now points at the generated reference for the full flag surface, so the flags are described in one place.

This is internal docs tooling, so there is no changelog fragment. The docs build passes under -W, with the directive rendering every subcommand.

[codspeed-hq]

Merging this PR will not alter performance

✅ 91 untouched benchmarks
⏩ 18 skipped benchmarks¹

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_{Comparing gaborbernat:docs/cli-reference (fa0df41) with main (6496744)}

Footnotes

1. 18 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩