[doc] add mkdocs doc

[reidliu41]

• add mkdocs doc for helping setup mkdocs env

[github-actions]

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[DarkLight1337]

I think it's better to simply expand the docs in https://docs.vllm.ai/en/latest/contributing/index.html#building-the-docs

[reidliu41]

sorry, miss that part, thanks for your remind.

[hmellor]

Thanks for doing this! A couple of comments that could be actioned in a follow up

[hmellor]

Could use an admonition here

Suggested change

	> **Note:** Ensure that your Python version is compatible with the plugins (e.g., mkdocs-awesome-nav requires Python 3.10+)
	!!! note
	Ensure that your Python version is compatible with the plugins (e.g., mkdocs-awesome-nav requires Python 3.10+)

[hmellor]

Suggested change

	Confirm that MkDocs is correctly installed::
	Confirm that MkDocs is correctly installed:

[reidliu41]

thanks, updated in #18996