[Tune] [Doc] Tune checkpointing and Tuner restore docfix

[justinvyu]

Why are these changes needed?

1. The docs suggest that a newly created Tuner will try to resume from the last checkpoint if an experiment of the same name is found at <local_dir>/<exp_name>. This was the case in the old tune.run API example since we specified resume="AUTO", but the new Tuner API always starts a new experiment. See attached issue for more details.
2. It was very hard to find how to actually save and load checkpoints. I added references to the Trainable documentation in the Tune checkpointing user guide. Previously, the checkpointing user guide didn't actually tell the user how to checkpoint: it only gave examples of how to configure CheckpointConfig and SyncConfig.

Related issue number

Closes #28722

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 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
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[maxpumperla]

Couple of nits, but this looks really good!

[xwjiang2010]

Really love this edition. The modularized sections look good to me.
Seems like some merging conflicts and linting errors to resolve?

[richardliaw]

Why break this out into a separate file?

[justinvyu]

I reuse this section in the Tune "working with checkpoints" user guide, since that's where I would intuitively look for an example of how to actually checkpoint. I've commented where that is below.

[richardliaw]

Can you instead link it, instead of re-rendering it in two places?

Otherwise for example, the search results are going to get cluttered.

[richardliaw]

So concretely, don't break it out into a separate file, put it in the "working with checkpoints" part, and use a relative reference here linking to that "working with checkpoints" section

[justinvyu]

Here are the examples again within the Tune user guide.

addressed!