Add DOAP files docs
Docs are missing info on doap files. This should take into account Apps for GNOME requirements.
See
- https://wiki.gnome.org/Git/FAQ#How_do_I_add_a_description_to_the_git_web_view.3F__What_is_this_.22blah.doap.22.3F
- https://blogs.gnome.org/sophieh/2021/08/15/apps-for-gnome-on-its-mark/
cc: @sophie-h