Rename genericgeometrypage to referenceelementspage

And move it from the special genericgeometry documentation directory
to the normal module documentation directory.
Remove doxygen page genericgeometrytransitionpage

This pages explains how to port code to the reference elements constructed
from tensor and conical products (i.e., the 'generic' reference elements).
It is very old and outdated, and only serves to confuse people nowadays.
Remove one tab.  This make images appear again.

Don't ask me why.
[cleanup,documentation] Remove the 'generic' prefix from various doxygen labels
[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.