Document group/role creation including user+permissions

[fenekku]

Is your feature request related to a problem? Please describe.

As more and more people run InvenioRDM they find the need to create internal groups with specific permissions to manage their instance. There isn't really a single place where this is documented currently (https://inveniordm.docs.cern.ch/releases/v12/upgrade-v12.0/#new-roles + discord are not discoverable enough).

Document that.

[ntarocco]

Done: https://inveniordm-dev.docs.cern.ch/operate/customize/users/#create-and-assign-roles