[docs] Text to speech task guide
This PR adds a multimodal task guide on fine-tuning SpeechT5 for text-to-speech. It's based on a wonderfully [detailed notebook](https://colab.research.google.com/drive/1i7I5pzBcU3WDFarDnzweIj4-sVVoIUFJ#scrollTo=uELTb9CcOaCp) by @hollance.
 
_The documentation is not available anymore as the PR was closed or merged._<|||||>PR with images: https://huggingface.co/datasets/huggingface/documentation-images/discussions/86