Alexander Kriegisch
a67fd6fe0d
Replace docs GIFs by PNGs
Follow-up on previous rename-only commit. This time, the binaries are
real PNGs. The asciidoc and other references to them have also been
replaced. File figures_classes.png in the teaching materials also was
rotated by 90° to display it correctly.
Background: The Asciidoctor PDF back-end cannot handle GIFs directly
without an additional Ruby Gem. It is easier to only use PNGs and JPEGs.
Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>