[Docs] Benchmark docs
This PR updates the docs for benchmarks and adds a README.md where the community can post their benchmark results. 

Would be happy about Feedback from @sgugger and @LysandreJik .

@LysandreJik - I deleted the part about "This work was done by [Timothy Liu](https://github.com/tlkh)." because the links were broken.
Actually will make a separate PR for the examples README.md - to have the  docs in v3.0.0.<|||||>@sgugger right now it's not ignored, so the slow test will fail because the output isn't the same. I don't think it's too big of a deal though, we can fix that after the release with only partial testing of the file.<|||||>Thanks for the review. Addressed them and also renamed the classes for consistency.