docs: add documentation about all cache files used by falcoctl

[loresuso]

What would you like to be added:
It would be nice making clear which files are used for caching purposes, like indexes.yaml, the indexes folder, the files used to store creds and client credentials and so on.

Why is this needed:
This is needed to let users better understand how falcoctl works.

[loresuso]

cc @therealbobo