Write up a small user manual

[Gerrit91]

References #57.

For the user section, explain:

• CLI
• Setup CLI
• Machine creation
• Project creation
• IP / Network creation
• Firewall creation
• ...
• How to add IP address to machines manually and how to peer with FRR (references Add section for user manual regarding BGP peering, BGP anycast #53)
• User management (v2)