Inference for multilingual models
This PR updates the multilingual model docs to include M2M100 and MBart whose inference usage is different (basically forcing the `bos` token as the target language id). Let me know if I'm missing any other multilingual models that have a different method for inference! :)

Other notes:
- Moved this doc to the How-to guide section because it seems like it is more of an intermediate-level topic. 
- Changed a section heading because the ampersand wasn't being properly displayed in the right navbar:
![Screen Shot 2022-02-25 at 9 40 57 AM](https://user-images.githubusercontent.com/59462357/155762069-ef1314a3-2474-48f8-85db-36dc78234c77.png)


The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15836). All of your documentation changes will be reflected on that endpoint.