doc: Add a .desktop and .appdata.xml file for Cockpit

This allows the user to launch Cockpit from the shell and also makes it visible
to be installed in GNOME Software.

IMPORTANT: This is not anyware good enough for merging, but we can work through the FIXMEs if this is a feature you guys actually want. :)
This would be: ```http://localhost:9090```

Once #870 goes in ... if this goes in first, then it would be port ```1001```