Add translating guide
# What does this PR do?
Add a translation guide so users have all the information they need to (1) contribute to a language that's already being translated, or (2) start their own issue for translating into a new language.

# Next step
Create a Translation Template for new issues (for example, [this template for Portuguese](https://github.com/huggingface/transformers/issues/16824) with all the docs that should be translated). I can do this.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the comments @sgugger! 

I moved the `_toctree.yml` tip to a part where it would be more relevant. Please let me know if you would prefer it in another part 🤗<|||||>LGTM!