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.)