Docs - Guide to add a new TensorFlow model
# What does this PR do?

Adds a guide on how to add a new TensorFlow model (architecture and weights), as well as a few tips to debug TF<>PT mismatches. This means that we can stop relying on ad hoc tips and instructions to add TF models, and point towards this guide instead 🙌 

I've also updated the test paths in the guide to add new models, which were outdated, and added a cheeky "from" in the "Converting from TensorFlow checkpoints" guide title to clearly distinguish it from the guide I'm adding in this PR.

Review suggestion: read the [live docs](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19256/en/add_tensorflow_model) themselves :)
_The documentation is not available anymore as the PR was closed or merged._<|||||>Awesome! I will merge the PR soon and make some guide-related announcements next Monday 🙌 