doc: Fix the 'make render-images' target

Closes #527
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
Use port 1001 by default to connect to cockpit-ws

Port 1001 is so much easier to type and remember. It's unassigned and
below 1024.

Closes #526
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>