Document the interface for editing network connections

It is possible to edit special configuration for each recorded network
connections. But this interface is not straightforward to find,
especially in GNOME 3.

It would be good to document that in maybe
/doc/anonymous_internet/networkmanager/ and
/doc/first_steps/persistence/configure/.

Feature Branch: doc/7185-network_connections

Related issues

Related to #7165

Original created by @sajolida on 7185 (Redmine)

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