Add some more text and links to existing documents.
I also remove 3 files which are now either outdated or whose contents is
also written (with more or less similar text) in other more up-to-date
files.
Add more links to other files after I reviewed they were still relevant.
The `gitlab-milestones.txt` in particular had to be updated because the
contents was outdated (though we still need to manage milestones, simply
now we are a bit more fine-grained).
Loosely based on the old structure.xml, except it was widely outdated.
So I removed or updated what was obsolete and added missing folders.
Obviously getting rid of the old `structure.xml` (now we have easier doc
generation through Gitlab).
Finally, I fix the table of contents and replaced the title with some
metadata-style stuff which Gitlab docs suggest (otherwise the document
title ends up in the table of contents, which is a bit silly).
This will be the root page for the developer documentation. Note that
there are other files in this directory (old `README` included) which
will need to be deleted but I don't do it just yet on purpose until I
checked them and integrate anything which could be of interest back into
the new documentation.
Jehan