docs: set date to SOURCE_DATE_EPOCH if available

docs: set date to SOURCE_DATE_EPOCH if available

Use the solution described in [1] to replace the call to the 'date'
command with a version that uses SOURCE_DATE_EPOCH if available. This
is needed for reproducible builds.

[1] https://reproducible-builds.org/docs/source-date-epoch/

Signed-off-by: Maximilian Engelhardt <maxi@daemonizer.de>
[Hans van Kranenburg]
Note: this patch is submitted upstream but not committed yet. We
expect that it gets in. Otherwise, we don't wait and already have it
here because I want to have the reproducible build work completed.

Gbp-Pq: Name 0040-docs-set-date-to-SOURCE_DATE_EPOCH-if-available.patch