Document New Group Utilities

[chrisjsewell]

Once the new group based capabilities are finalized, principally #3613, #3882, #3903,
we should update the documentation, e.g. docs/source/working_with_aiida/groups.rst,
with a guide for users.

[chrisjsewell]

I notice now that there some additions to this in #3882, but certainly for #3613 there is nothing yet