[docs] Trainer docs

[stevhliu]

Part 2 of #27986 to finish cleaning up the Trainer API docs. This includes:

• moving the CUDA extension installation problems to the performance and scalability debugging doc where it is more appropriate
• GPU selection has its own section in the multiple GPU training doc
• spin out the FSDP sections into their own docs
• add a link from the Trainer guide to the FSDP guide

[amyeroberts]

Thanks - looking good 😎!

Could you run pytest tests/utils/test_doc_samples.py to make sure everything passed before merging?

[stevhliu]

Test is passing!

I rebased to incorporate the changes from #28135, but that didn't work for some reason, so I manually edited tests/utils/test_doc_samples.py to reflect the latest changes. Is this still ok since the change already exists on main?

[amyeroberts]

@stevhliu Yep - it should be fine!