Update plugin development documentation to prefer NautobotUIViewSet approaches

[glennmatthews]

Currently the Adding Web UI Views section of the plugin development documentation first introduces creation of generic Django views, then introduces Nautobot's generic view classes and their approach to templates and URL registration, and only third introduces NautobotUIViewSet and its usage. We should be strongly encouraging new plugin development to use NautobotUIViewSet; personally I'd vote for removing the generic Django and generic Nautobot views entirely from the forward-looking plugin dev docs, but at the very least NautobotUIViewSet should be the first approach introduced.