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.

Fixes #1006
Reviewed-by: Stef Walter <stefw@redhat.com>
doc: Added screenshots and contact info to appdata

Closes #1085
Reviewed-by: Stef Walter <stefw@redhat.com>
doc: Add some new built files to .gitignore