Documentation about loading a fast tokenizer within Transformers
This PR does two things:
- Allows to load a fast tokenizer from an instantiated `tokenizers` object
- Adds a page to document how to use these tokenizers within `transformers`

See [here](https://190138-155220641-gh.circle-artifacts.com/0/docs/_build/html/fast_tokenizers.html) for the generated docs
Thanks for the suggestion @sgugger!