docs: Add HSTU guide to Getting Started toc

[yinggeh]

What does the PR do?

Adds a Getting Started table-of-contents entry for Hierarchical Sequential Transduction Units (HSTU) / Generative Recommenders, linking to the tutorials HSTU guide (Torch AOTI serving path with pointers to recsys-examples).

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
• Verified copyright is correct on all changed files.
• Added succinct git squash message before merging ref.
• All template sections are filled out.
• Optional: Additional screenshots for behavior/output changes with before/after.

Commit Type:

Check the conventional commit type
box here and add the label to the github PR.

• build
• ci
• docs
• feat
• fix
• perf
• refactor
• revert
• style
• test

Related PRs:

Part of the HSTU documentation set (suggested merge order: tutorials → pytorch_backend → server):

• tutorials: HSTU Popular Models guide — docs: add HSTU (Generative Recommenders) tutorial page tutorials#159 (land first so the toc link resolves)
• pytorch_backend: AOTI see-also link to the HSTU guide — docs: Link HSTU AOTI example from PyTorch backend README pytorch_backend#201

Where should the reviewer start?

• docs/contents.rst (Getting Started toctree entry)

Test plan:

• Confirm the new toc entry points at ../tutorials/Popular_Models_Guide/HSTU/README.md

• After the tutorials HSTU guide PR merges, verify the docs site Getting Started section lists HSTU (Generative Recommenders) and the link resolves

• CI Pipeline ID:
  N/A (docs-only toc change)

Caveats:

• The linked tutorials page must exist (or be merged) for the published docs link to resolve. Prefer merging the tutorials guide before or with this PR.

Background

Triton can serve HSTU generative recommender models via the PyTorch backend (platform: "torch_aoti"). This toc entry surfaces that path next to other popular-model Getting Started links.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Relates to accompanying tutorials (Fix front-ends to stop correctly. #159) and pytorch_backend (Avoid warnings when metrics aren't initialized #201) documentation PRs for HSTU / Torch AOTI