Add tutorial doc for TF + TPU
This is the sidebar tutorial for training with TF + TPU, to go with [the code notebook](https://github.com/huggingface/notebooks/pull/313).

Note that the Markdown is exported straight from Notion, so some formatting will probably look very wrong - I'm working on cleanup!
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sayakpaul The spaces have been removed because the extra content after them got moved to a whole other doc!