user-guides: document user setup

[lucab]

There are few dedicated options in FCCT/Ignition to manipulate GECOS user, but their documentation is a bit spotty. We can start from https://coreos.com/os/docs/latest/adding-users.html, polish that a bit, and add more FCCT examples.

Things I have in mind:

• mention the default (privileged) administration user core
• basic customizations to core: SSH keys, groups
• how to add new users/groups
• password generation and customization
• sudo setup

[adriannemora]

I just stumbled on this now, as I'm still learning how to navigate github. I did mention core and explained how to add users and groups to the fcc file. I am adding a link to https://docs.fedoraproject.org/en-US/fedora/f31/system-administrators-guide/basic-system-configuration/Managing_Users_and_Groups/ for more basic instructions about to create and manage users and groups.

[jlebon]

We could rename the Authentication page to "Configuring Users", and expand that page instead.