+ activate the devhelp extension and update the build-depends
+ use tar-ustar to allow us to have filenames longer than
  99 characters in archives.
+ use -Wno-portability in order to use the hotdoc Makefile

The text for the standalone markdown documentation was taken
from the gtk-doc files removed in a previous commit.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Mathieu Duponchelle <mathieu.duponchelle@opencreed.com>
Differential Revision: https://phabricator.apertis.org/D4137