DOC: add basic usage of filesystem

[erfannariman]

Would it be valuable if basic usage documentation was extended with methods to navigate and check a directory? Think of methods like ls, glob, info (and more) and also how we can use the filter argument.

If it is already documented somewhere and I missed it, feel free to close this issue.

btw, I am willing to make a PR to add this to the documentation.

[martindurant]

More usage documentation is certainly welcome! https://filesystem-spec.readthedocs.io/en/latest/usage.html#use-a-file-system is essentially what we have right now, and I suppose we are relying on users guessing method names based on their experience with terminals.

[martindurant]

To be clear: would be very happy to see a PR with updated usage docs!