Add some info about FastPitch SSL model
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Adds a paragraph with some basic information about the SSL FastPitch model. Will add more as the model is finalized and the paper released.

**Collection**: TTS

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

# Additional Information
* Related to #5948 
