Use user-specified custom cache directory

[pllim]

As @bsipocz asked in #5828 (comment), "I run into this again today, and it seems we can change the config dir, but not the cache dir? If I misunderstand the situation, then we probably need to add more examples to the config dir, one especially for the case of the custom cache dir."

There is an option to change cache directory but has to be via XDG_CACHE_HOME. However, on Linux, changing it affects system-wide setting.

Maybe @bsipocz can clarify if this is a matter of documentation of improving https://docs.astropy.org/en/latest/config/index.html#astropy-config and/or https://docs.astropy.org/en/latest/utils/data.html . Or XDG_CACHE_HOME is simply insufficient.

Might also be relevant: #6803

Previous unmerged attempts: #5828 #6985