Add transformers MMS checkpoints to docs

[patrickvonplaten]

Before submitting

• [x] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [x] Did you read the contributor guideline?
• [x] Did you make sure to update the docs?
• [ ] Did you write any new necessary tests? No just docs

Just a PR to add Transformers' MMS implementation to the docs