pgsql: docs: html: copy images to output as part of xslt build

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0012979d989fd1640ed17c32b303ebbd2294a3fc

Modified Files
--------------
doc/src/sgml/Makefile              |  1 -
doc/src/sgml/stylesheet-common.xsl | 22 ++++++++++++++++++++++
doc/src/sgml/stylesheet.xsl        |  6 ++++++
3 files changed, 28 insertions(+), 1 deletion(-)