[TTS] Add NeMo TTS Primer Tutorial
# What does this PR do ?

Add a primer document to our TTS tutorials covering general introductory material related on models and running them in NeMo.

**Collection**: TTS

# Changelog 
- Add TTS Primer notebook
- Add images directory (similar to ASR tutorials)
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [x] Documentation