[docs] Performance docs tidy up, part 1 
The [performance and scalability section](https://huggingface.co/docs/transformers/v4.29.1/en/performance) of the docs is difficult to navigate and find actionable advice in, according to feedback. Some topics are missing too. As part of the 2023 docs roadmap, we planned to refactor the section to make the actionable bits easier and quicker to find. 

The refactor will come in several PRs to split it into more manageable chunks. This is the first part where you'll find: 
- reworked Overview for easier navigation
- reworked "Training on a single GPU" part. This part has been reorganized to have a clearer structure, the conceptual content has been clarified, the how to use/enable examples made visible, while the benchmarking comparison has been removed (it bloats the doc), instead there are links to benchmarks. 
Overall, I think that this section should now be easier to navigate and find actionable pieces with reasonable amount of explanation left in the doc, and relevant links for more information. 
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Also cc @lvwerra and @stas00 who contributed the original guide.

Thank you for the ping, Sylvain.

@lvwerra and I had two very different approaches on how to do the new documentation, and since we went with Leandro's way I will let Leandro comment on these changes.<|||||>Sorry about delay. I have moved the "Model training anatomy" part into a separate conceptual guide, and linked the two docs. This should address most of the concerns voiced in this discussion. Other feedback has also been incorporated. Please take another look: cc @lvwerra @sgugger <|||||>Would you like to take another pass on this PR, @lvwerra ?