Add information about all public dirs/files in CONAN_USER_HOME

It would be nice to have the following
- list of all (user-related) directories and files in CONAN_USER_HOME with corresponding description.
- a subset of files above that is affected by `conan config install` (thus could be shared)

The best place for this, IMO, would be https://docs.conan.io/en/latest/reference/commands/consumer/config.html .

Reason: this will allow conan users to quickly see all the things that could be shared with `conan config install`. E.g. `templates` subdir is only mentioned in [conan new](https://docs.conan.io/en/latest/reference/commands/creator/new.html) and I've only found out about it by accident.