This is an attempt of breaking up the documentation, currently spread across
several pages. We're left with a few high-level topics with various things
grouped underneath those.
Further refinement is necessary, but this can now be done in incremental steps
over massive reworks.
While doxygen can handle markdown pages, support for it is very limited:
markdown pages can only be included as a whole page, they get an automatic
title (custom titles are possible but aren't standard markdown) and it's not
possible to use \subpage without messing with the markdown again. Any markdown
page will thus end up as separate item in the doxygen output, not really
suitable for generating a good page hiearchy.
Let's switch the tutorial to use doxygen directly instead of markdown, short
of using code/endcode instead of markdown's ``` there isn't that much
difference anyway but it allows us to structure things nicer in the online
docs.
