Add a DEP-14 diagram

The diagram comes in two formats:
 - SVG with editable text (apertis-dep-14-gitlab.svg)
   This is the primary source.
 - SVG with texts as paths (apertis-dep-14-gitlab-curves.svg)
   This is generated from the primary source; it is a preferred
   format for embedding as a figure in a document, since it
   doesn’t depend on the availability of fonts or the renderer’s
   ability to render them correctly.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

content/guides/gitlab-based_packaging_workflow.md

@@ -20,6 +20,8 @@ The packaging git repositories follow the
 [DEP-14](http://dep.debian.net/deps/dep14/) Git layout specification with the
 following conventions:
 
+![DEP-14 in Apertis](/images/apertis-dep-14-gitlab-curves.svg)
+
 - `upstream/${UPSTREAM_DISTRIBUTION}` branches with the unpacked upstream
   project code from the debian package (e.g. `upstream/buster`)
 - `debian/${UPSTREAM_DISTRIBUTION}` branches with the Debian changes on top of