Store cache under user's cache folder
Cached content should be stored under the user's cache folder instead of the current directory (or any other folder in the project). Using this central location defined by the user's preferences allows the user to more easily locate and manage these caches. And Antora doesn't litter folders all over the place.
The XDG base directory specification defines this location. Where XDG isn't used, fallback to other well-known locations. See https://github.com/atom/atom/issues/8281 for a discussion and more details.
One of following two packages could be used to locate this folder:
It should also be possible to override this location by passing the --cache-dir
CLI option or the ANTORA_CACHE_DIR environment variable.
Make sure to provide an override in tests so that the real cache folder doesn't get inadvertently cleaned.