Add method to push adapters to HF Model Hub

[calpt]

Follow-up to #162.

Similar to push_to_hub() in transformers, this PR adds a new method push_adapter_to_hub() that allows pushing a loaded adapter by name, e.g.:

model.push_adapter_to_hub(
    "my-awesome-adapter",  # repo name
    "awesome_adapter",  # adapter name
    adapterhub_tag="sentiment/imdb",
    datasets_tag="imdb"
)

One of the two args adapterhub_tag and datasets_tag is required, so we can automatically pull adapters from huggingface.co to AdapterHub.ml with correct categorization.

Also added a new doc page (adapter_docs/huggingface_hub.md) for describing the HF Model Hub integration.

cc @osanseviero just fyi :)

[osanseviero]

@julien-c @LysandreJik Look this great stuff 🤩

[LysandreJik]

That's great @calpt!! Love the source=hf!

And very nice that you're offering the possibility for users to have a local directory to sync, rather than the temporary directory which would mean re-cloning every time!

[calpt]

Thanks for the feedback @osanseviero @LysandreJik !