Improve docs for modules

The modules' docs are a bit messy at the moment — there are no sections/categories, and I always end up using CMD+F to search for what I am looking for:

https://just.systems/man/en/modules1190.html

Start with the modules documentation at https://just.systems/man/en/modules1190.html and review how its content is currently organized. Group the material into clear sections or categories so readers do not need to rely on CMD+F. Done means the modules documentation has a coherent structure that makes topics easier to find.

Written by the indexing model from the issue text.