docs: html: copy images to output as part of xslt build
Until now the meson built HTML docs had non-working references to images. They
were copied in the make target. Instead of duplicating that for meson, copy
them as part of the xslt stylesheet.
Reported-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/
3fc3bb9b-f7f8-d442-35c1-
ec82280c564a@enterprisedb.com