Make documentation builds reproducible

Somewhere at PGCon, I forgot exactly where, maybe in the same meeting
where we talked about getting rid of distprep, we talked about that the
documentation builds are not reproducible (in the sense of
https://reproducible-builds.org/). This is easily fixable, the fix is
available upstream
(https://github.com/docbook/xslt10-stylesheets/issues/54) but not
released. We can backpatch that into our customization layer. The
attached patch shows it.

I had actually often wanted this during development. When making
documentation tooling changes, it's useful to be able to compare the
output before and after, and this will eliminate false positives in that.

This patch addresses both the HTML and the FO output. The man output is
already reproducible AFAICT. Note that the final PDF output is
currently not reproducible; that's a different issue that needs to be
fixed in FOP. (See
https://wiki.debian.org/ReproducibleBuilds/TimestampsInPDFGeneratedByApacheFOP.)