Improve performance docs
# What does this PR do?

As discussed with @stas00 this PR does the following things:
- adds files for missing sections so contributors can better find the place to add content.
- adds a disclaimer at the beginning stating that a lot of general training information is in the single GPU training section
- fixes the link of CPU inference

Looking at the ToC I was wondering whether we should add subsections like it is done for the tasks to make the main ToC a bit slimmer. Otherwise we have the main performance docs (the entry point) there plus all the other sections (~8-10). What do you think?

## Before submitting
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- [ ] Did you write any new necessary tests?
_The documentation is not available anymore as the PR was closed or merged._<|||||>Sounds good @sgugger, I added the "Coming soon" and restructured the ToC.<|||||>Communications are difficult. 

what I proposed is to add normal documents - not "Coming Soon"

inside those documents they should all explicitly point the user to read the first document that is already filled out and that we will expand this other currently mostly empty doc with the missing material.

In other words e.g,, perf_infer_gpu_many.mdx should have:

1. read perf_train_gpu_one.mdx first
2. this document will be completed soon.

If the docs remain "Coming Soon" nobody will read them and will miss out on the already rich performance docs we have.

And the impetus for this change was that we went from a complete solution - all performance notes in one doc, to very incomplete solution, making it look like we only have advise for those with 1 gpu and doing training.

The original discussion back in winter was that all performance docs will be filled out, but it was dropped after the first document and no signs of new docs coming any time soon. So this proposal was my attempt to rescue the situation.<|||||>Ok I added references to each document and a small text explaining what will come there. Is that what you had in mind @stas00?