Looking glass is not just helpful for development, but also when
asking users to provide relevant information.
Having it in-tree makes it easy to reference, and ensures that
the documentation is still in a maintained place when plans to
retire the wiki go head.
The content is based on the [wiki-page] with updated code samples,
small sections on added features, and outdated references removed
(anyone remember project looking glass?).
[wiki-page]: https://wiki.gnome.org/Projects/GnomeShell/LookingGlass
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2932>
