[Help wanted] Add doc about SupertonicTTS models

[csukuangfj]

We need to update
https://k2-fsa.github.io/sherpa/onnx/tts/supertonic/index.html
to describe how to use SupertonicTTS models.

Please see also

• Add Supertonic2 TTS support #3094
• Upload supertonic tts models #3263
• Add Python API examples for Supertonic TTS #3264