Missing docs for bench CLI and API

[nickserv]

Clear and concise description of the problem

As far as I can tell, we haven't added any CLI or API docs for vitest bench and bench() yet.

Suggested solution

Document the benchmarking tool like we document other parts of the test runner.

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[FINDarkside]

I think the docs are still somewhat lacking, for example it seems like there's some pattern to match bench.ts files but I couldn't find any mention of it in docs. I just guessed since it seemed intuitive.