[Docs] Fixing warnings, focusing on SpeechLLM docs + copy editing
# What does this PR do ?

Various fixes to docs to reduce the warnings we currently have and make sure docs work as intended.

Requesting reviews from: 
* @yaoyu-33 - glance over docs fixes
* @stevehuang52 - check over type hint change (from using | to Union), see more info in changelog

**Collection**: Docs, SpeechLLM

# Changelog 
- Fixed broken link to SpeechLLM docs, so now they are actually accessible.
- Fixed errors we were getting from type hints. This is an issue @pzelasko spotted previously (https://github.com/NVIDIA/NeMo/pull/8956). There is a good explanation of it in the "note" section here: https://docs.python.org/3/library/stdtypes.html#union-type.
- Fixed some docstrings in SpeechLLM code so now they will display properly in the API section
- Various other small fixes

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