Make documentation builds reproducible

Make documentation builds reproducible

Currently, the documentation builds are not fully reproducible (in the
sense of https://reproducible-builds.org/).  A fix is available
upstream (https://github.com/docbook/xslt10-stylesheets/issues/54) but
not released.  This commit patches the upstream fix into our
customization layer.

This patch addresses both the HTML and the FO output.  The man output
is already reproducible.

Discussion: https://www.postgresql.org/message-id/flat/9077b779-a9f8-09c8-6e85-da1ebfba15af@eisentraut.org