Add DCP async checkpointing info to docs

[ankitageorge]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• [x ] update tests and/or documentation
• other (please add here)

Please link to any issues this PR addresses.

Changelog

What are the changes made in this PR?

• Add information about DCP async checkpointing to docs

Test plan

Please make sure to do each of the following if applicable to your PR. If you're unsure about any one of these just ask and we will happily help. We also have a contributing page for some guidance on contributing.

• run pre-commit hooks and linters (make sure you've first installed via pre-commit install)
• add unit tests for any new functionality
• update docstrings for any new or updated methods or classes
• run unit tests via pytest tests
• run recipe tests via pytest tests -m integration_test
• manually run any new or modified recipes with sufficient proof of correctness
• include relevant commands and any other artifacts in this summary (pastes of loss curves, eval results, etc.)

UX

If your function changed a public API, please add a dummy example of what the user experience will look like when calling it.
Here is a docstring example
and a tutorial example

• I did not change any public API
• [ x] I have added an example to docs or docstrings

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2837

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 2 New Failures, 2 Unrelated Failures

As of commit 9b4deb2 with merge base c00aa57 ():

NEW FAILURES - The following jobs have failed:

• Build Docs / build_docs (3.11) (gh)
  Process completed with exit code 2.
• GPU tests / gpu_test (3.10, stable) (gh)
  tests/recipes/test_eleuther_eval.py::TestEleutherEval::test_torchtune_checkpoint_eval_results[truthfulqa_gen-0.1818-4]

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• GPU tests / gpu_test (3.9, stable) (gh) (similar failure)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• GPU tests / gpu_test (3.11, stable) (gh) (trunk failure)
  ##[error]The operation was canceled.

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[joecummings]

Just one nit, but otherwise looks good!

[joecummings]

Suggested change

	_component_: torchtune.training.FullModelMetaCheckpointer
	_component_: torchtune.training.FullModelHFCheckpointer

Hf checkpointer is our preferred