[CLI] print help if no command provided

[Wauplin]

From feedback of @gary149 in DMs (private)

With the PR, if a CLI command is not complete, the corresponding help section will be printed to the terminal. Before that, only the root hf --help was printed.

Example with hf auth:

> hf auth
usage: hf <command> [<args>] auth [-h] {login,logout,whoami,switch,list} ...

positional arguments:
  {login,logout,whoami,switch,list}
                        Authentication subcommands
    login               Log in using a token from huggingface.co/settings/tokens
    logout              Log out
    whoami              Find out which huggingface.co account you are logged in as.
    switch              Switch between access tokens
    list                List all stored access tokens

options:
  -h, --help            show this help message and exit

(same for hf repo, hf cache and hf hobs)

[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]

failing tests are unrelated (HTTP 429) so time to merge :)