add Doc procedures of assigning zone of an interface via ifcfg & keyfile

The keyfile scenario required modifying of an existing .nmconnection file. This MR adds a step that implements this via python's ConfigParser in addition to other minor changes required.

@runtests:pass,doc_firewalld_zones_via_keyfile,doc_firewalld_zones_via_ifcfg
@os:rhel9.3