Update documentation structure and add psk docs

[lulf]

• Restructure admin guide a bit to align it more with the 'user guide' concept, since the scope of 'deployment' has gone beyond with monitoring and tracing, more day 2 operations docs.
• Add TLS-PSK and X.509 authentication docs to device management
• Add general section on authentication
• Reword some places where KNative is no longer used.