FastPitch FT notebook - Improving Speech Quality clarifications
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?
Clarifications to the FastPitch fine-tuning notebook regarding how to improve speech quality by generating synthesized mels; added missing instructions for adding more data/multiple speakers for FastPitch.

**Collection**: TTS

# Changelog 
- Moved synthesized mel code out of text block and into a separate code block, fixed typos s.t. it runs
- Added missing info for fine-tuning FastPitch on multiple speakers (need to update `sup_data_types`)
- Additional info about mixing speakers
- Other minor text updates to the notebook

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

# Additional Information
* Related to #3678 and #3895
