Add push_to_hub docs
Looks good to me! :)

Maybe we can mention that users can also set the `private` argument if they want to keep their dataset private? It would lead nicely into the next section on Privacy. Thanks for your comments, I fixed the capitalization for consistency and added an passage to mention the `private` parameter and to have a nice transition to the Privacy section :)

I also added the login instruction that was missing before the user can actually upload a dataset.
Since #3098 it's now possible to upload a dataset on the Hub directly from python using the `push_to_hub` method.
I just added a section in the "Upload a dataset to the Hub" tutorial.

I kept the section quite simple but let me know if it sounds good to you @LysandreJik @stevhliu :)