Add documentation for Groups API

Add docs describing how to create, list, update or delete groups and
subgroups (companies and departments respectively).

Docs also describe adding new members or removing existing members from
a group and subgroup.
Add how to delete an attachment

This is already working in the API but was not documented.
Merge branch 'update-documentation-add-groups-and-organizations'
Merge branch 'update-delete-attachment-docs'