[HfFileSystem] Add top level hffs

[lhoestq]

>>> from huggingface_hub import hffs
>>> with hffs.open("datasets/fka/awesome-chatgpt-prompts/prompts.csv", "r") as f:
...     print(f.readline())
...     print(f.readline())
"act","prompt"
"An Ethereum Developer","Imagine you are an experienced Ethereum developer tasked..."

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[Wauplin]

Thanks! Maybe mention it in https://huggingface.co/docs/huggingface_hub/guides/hf_file_system as well? I would make the default snippets use hffs and mention that instantiating HfFileSystem is possible

Note: failing CI is unrelated (will be fixed in https://huggingface.co/docs/huggingface_hub/guides/hf_file_system)