Docs visual improvements

[Ssofja]

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What does this PR do ?

Improves the docs top-level sidebar navigation by unwrapping nested sections so collections, tools, and APIs are visible without an extra click, reordering sidebar sections, and reordering collection entries.

Collection: Documentation (top-level docs navigation; affects all collections)

Changelog

• docs/source/index.rst
  • Unwrapped Collections — list collection intro pages directly instead of a single "NeMo Collections" wrapper
  • Unwrapped Speech AI Tools — list individual tool pages directly instead of tools/intro
  • Unwrapped APIs — list core and collection API pages directly instead of apis
  • Moved Collections above APIs; moved APIs to the bottom of the sidebar
  • Collection order in sidebar: ASR → TTS → SpeechLM2 → Diarization → Speaker Recognition → Speech Enhancement (audio/intro) → SSL → Speech Classification
• docs/source/collections.rst
  • Updated collection order in the hub page to match the new sidebar order
  • No changes to dataloaders — left as merged in Refactored all ASR collections documentation #15542 (after asr/intro)

Usage

N/A — documentation navigation change only. Preview locally:

pip install -r requirements/requirements_docs.txt
make -C docs html
open docs/build/html/index.html

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• New Feature
• Bugfix
• Documentation

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Additional Information

• Docs-only change; no code or test changes required
• Hub pages (collections.html, apis.html, tools/intro.html) remain in the repo for existing links but are no longer the primary sidebar entry points
• dataloaders.rst and its placement in collections.rst are unchanged from Refactored all ASR collections documentation #15542

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