Rewrite setup_configuration documentation with YAML example directive

[stevenbal]

Clear up any TODOs currently in the docs and add a reference to the django-setup-configuration docs for more info on how to use the command (e.g. https://github.com/open-formulieren/open-forms/blob/feature/4882-document-setup-config-steps/docs/installation/setup_configuration.rst)

Also use the setup-config-example directive to document the configuration steps. It may also be useful to use autoclass to display the docstring of the steps for a general description per step.