Fix push_to_hub docstring and make it appear in doc
# What does this PR do?

As pointed out in #12700, the documentation of `psuh_to_hub` was not appearing in the objects like `PretrainedConfig`, `PreTrainedModel` or `PreTrainedTokenzier`. This PR addresses that. It also customizes the documentation for each object (in particular the example) but since I didn't want to make (and maintain) five copies of the docstring, I'm reusing the technique we use for Flax docstrings (copy the method and then adapt its docstring). I think it's okay to have a refactored approach here since it's for the internals of the lib but let me know.

This PR also simplifies the doc page of the tokenizer by using the `special_members` keyword for the methods we want to force to appear even if they are not redefined in the subclass, instead of naming everything in `members`.

Fixes #12700
