Add a quick start guide

[stevhliu]

Currently, the docs don't have an end-to-end guide demonstrating how users can string everything together in the huggingface_hub library. There are three guides, and each of them highlight a different aspect of the library. I think adding a quick start guide would add value for users by capturing the core parts that are representative of the library (namely downloading, uploading, and creating/managing files and repositories).

For the quick start guide, I think we should start with the simplest option:

1. hf_hub_download for downloading files instead of the two-step hf_hub_url and cached_download wombo combo.
2. create_repo to create a repo.
3. upload_file to upload files.

Would love to hear if you think there is anything else we should include in the quick start or any other feedback, thanks :)

[adrinjalali]

also related to #824

[stevhliu]

Closed by #850.