[Doc] Improve benchmark documentation

[rafvasq]

This PR tries to improve the documentation regarding vLLM performance including benchmarking vLLM where information can be found in multiple places (1, 2)

• "Performance" header instead of "Performance Benchmarks" to accommodate future performance-related doc
• Include more overview information on the website docs, pointing to the github docs for more information

A possible follow-up to this PR would be to clean up the docs found in the repo assuming that's where the detailed, technical information will live, otherwise it should get pulled out for the website docs.

[github-actions]

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[russellb]

The changes lgtm overall.

The one thing I'd change if you have to update this anyway is to try to get git to recognize benchmarks.rst as a move instead of a delete + create. As it is, you lose history of the file. Generally git mv will do what you want.