[wip][train] update Ray Train documentation (quickstart, user guides)

[matthewdeng]

Why are these changes needed?

Rendered docs are here: https://anyscale-ray--38287.com.readthedocs.build/en/38287

This makes a few iterative changes to the Ray Train documentation.

1. Add Getting Started with PyTorch and Getting Started with PyTorch Lightning pages.
   1. Labeled as PyTorch and PyTorch Lightning in the Table of Contents.
2. Rename Distributed PyTorch to User Guides.
   1. Updated the page to have a Table of Contents of the different guides.
3. Move HuggingFace content into a HuggingFace Transformers & Accelerate page.
   1. This is temporary until we have dedicated pages for these.
4. Move the LightningTrainer Migration Guide content directly into the Getting Started with PyTorch Lightning page.
5. Remove the content from Distributed PyTorch and Converting an Existing Training Loop in favor of the above pages.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[krfricke]

Generally big fan of this.

For naming, the current layout renders for me like this:

This is not ideal as the distinction between "PyTorch" and "PyTorch Lightning" guide is hard to see ("Lightning" is in the next row).

1. Should we use "Native PyTorch" or (vanilla/raw/pure) or something like that instead?
2. Let's start with the framework, e.g. "PyTorch starter guide", "PyTorch Lightning starter guide", "Huggingface Transfomer starter guide" or so?

[woshiyyya]

Left some comments. I love these Getting Start Pages. This is very succinct and can be the the major entry point for new users. (Getting Start -> User Guides / API Page)