Document Auth features

[satterly]

• Use guest scope as default alerta#1344
• Support user-defined list of user roles alerta#1346
• Unable to have guest as default role alerta#1414
• feat: support readonly users alerta#1505
• remote_user authentcation via proxy alerta#1578
• X-Forwarded-User http header alerta#1615

[satterly]

Auth features are now documented in configuration.rst and authentication.rst. Guest scopes, user-defined roles, readonly users, proxy auth, and CAS auth are all covered.