Expanded the introduction and possible areas of contribution. Also
improved structure and added more links to communication channels.
Removed the description of how patches can be submitted, in favour
of a more prominent link to welcome.gnome.org, which provides this
information in much more depth.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/855>
This page will be replaced by the two pages created in the previous
commits, new-local-calendar and new-online-calendar.
To keep the project compiling the button to add a new calendar now
temporarily points at new-local-calendar. The button will be replaced
by a menu exposing both new pages in one of the following commits.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/850>