Unify benchmarking and profiling docs (#5145)
This moves all docs about benchmarking and profiling into
CONTRIBUTING.md by moving the readme of `ruff_benchmark` and adding more
information on profiling.
We need to somehow consolidate that documentation, but i'm not convinced
that this is the best way (i tried subpages in mkdocs, but that didn't
seem good either), so i'm happy to take suggestions.
README.md