docs: html: load stylesheet via custom.css.source

docs: html: load stylesheet via custom.css.source

Until now the meson built docs did not have a working reference to the css
stylesheet, it was copied in the make target. Instead of duplicating that for
meson, use the docbook-xsl parameter custom.css.source to reference it. An
additional benefit of that approach is that the stylesheet is now included in
the single-file HTML documentation.

Reported-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/3fc3bb9b-f7f8-d442-35c1-ec82280c564a@enterprisedb.com