Now that we have a collection of scripts around the toolbox
workflow, it makes sense to add a brief overview over the
available tools.
For now it's just located in the corresponding tools folder,
but it gives us something to point to once we overhaul the
toplevel documentation as part of the wiki migration.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3040>
