TTS model documentation update
TTS model documentation update

**A clear and concise description of what the bug is.**

The Readme's for the currently supported TTS models are outdated. E.g. there is no hint on that now VITS is supported, nor how SSL-Fastpitch is to be used and how it improves/deviates from the normal Fastpitch model. 

**Please list *minimal* steps or code snippet for us to be able to reproduce the bug**

Navigate to the TTS description: https://docs.nvidia.com/deeplearning/nemo/user-guide/docs/en/main/tts/intro.html. Search for VITS docuementation or SSL Fastpitch.


**Expected behavior**

Thorough documentation of the new language models or how to train and if possible current status and performance metrics.