Vits doc
# What does this PR do ?

Update documentation for VITS

**Collection**: [TTS]

  
**PR Type**:
- [ ] New Feature
- [ ] Bugfix
- [x] Documentation


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue) https://github.com/NVIDIA/NeMo/issues/5948
