docs, tests: replace datasets.rst with memory_optimizations.rst (#968)

datasets.rst was removed from torchtune repo.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

# What does this PR do?

Replace a missing 404 document with another one that exists. (Removed it
from
the list when memory_optimizations.rst was already pulled.)


## Test Plan

Please describe:
 - tests you ran to verify your changes with result summaries.
 - provide instructions so it can be reproduced.


## Sources

Please link relevant resources if necessary.


## Before submitting

- [x] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [x] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

docs/source/getting_started/index.md

@@ -185,7 +185,7 @@ client = (
 )  # or create_http_client() depending on the environment you picked
 
 # Documents to be used for RAG
-urls = ["chat.rst", "llama3.rst", "datasets.rst", "lora_finetune.rst"]
+urls = ["chat.rst", "llama3.rst", "memory_optimizations.rst", "lora_finetune.rst"]
 documents = [
     Document(
         document_id=f"num-{i}",