[cleanup] Move image files from genericgeometry/doc to central doc subdirectory

These images contain the images of the various references, used to explain the
subentity numbering.  The directory I move them to already contains some of
these images, but some of those still show the old numbering.  Others are
cropped badly.
(cherry picked from commit 60160f9f5d2f5bba65fcb109853c4421b0669e9d)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
Ignore the now-empty genericgeometry/doc directory
(cherry picked from commit 570666985d679d62c84d5bddb35d948e191e2b1d)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
[cleanup,documentation] Remove unused drawings of the reference elements

This drawings show the numbering of the reference elements from the
pre-generic-referenceelements-era.  They are obsolete, and not referenced
anywhere in the module.  This patch removes them.
(cherry picked from commit 1720ea33f0ba654ba3f3913be42d56a0bc1b15de)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>