Add full serve CLI reference back to docs

[hmellor]

Now that #20206 has merged. This PR reinstates the full vllm serve --help CLI reference that was unfortunately removed in the upgrade from Sphinx to MkDocs in #18145.

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[gemini-code-assist]

Summary of Changes

Hello @hmellor, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request reintroduces the comprehensive CLI reference for the vllm serve command into the documentation. It achieves this by centralizing the definition of serve command arguments and updating the documentation generation process to automatically include these details, ensuring the serve --help output is fully reflected in the MkDocs documentation.

Highlights

• Documentation Restoration: Reinstated the full vllm serve --help CLI reference in the MkDocs documentation, which was previously lost during a documentation system migration.
• Argument Definition Centralization: Moved the vllm serve command's argument definitions from vllm/entrypoints/cli/serve.py to vllm/entrypoints/openai/cli_args.py to centralize them for consistent documentation generation.
• Documentation Generation Improvement: Enhanced the generate_argparse.py script to dynamically create markdown for CLI arguments with configurable heading levels, ensuring accurate and flexible documentation output.

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[gemini-code-assist]

Code Review

This pull request is a great improvement, as it not only restores the vllm serve CLI reference to the documentation but also refactors the argument parsing logic for better maintainability. By centralizing the argument definitions, the code is now cleaner and less prone to inconsistencies.

I've identified a couple of areas for improvement in the documentation generation logic to ensure the output is both complete and as clear as possible for users. My suggestions are detailed in the review comments.

[gemini-code-assist]

The heading ### help is a bit generic and could be more descriptive for users navigating the documentation. A more specific heading would improve clarity.

Suggested change

	### help
	### `vllm serve` Arguments

[DarkLight1337]

I would call this section Options

[DarkLight1337]

Can you update configuration/serve_args.md to link to this page as well?

[kouroshHakha]

At high level, these belong to FrontEndArgs?

[hmellor]

I thought about adding them, but I wasn't sure about the data parallel ones though, because they are sort of "pre-frontend"