docs: Remove obselete appendix about parsing cli argumnets

It's a bad idea trying to mix the Options from GStramer and GTK, in addition with cli argument being a bit wonky thing for GUI applications in general. In the rare, now, occasion that an application wants to parse arguments, its preferable to parse them manually and use library apis afterwards rather than trying to combine the option groups and hope it works.

In addition, applications should be opening files using g_application_open instead of parsing random arguments.