doc/asciidoc: always use UTC

Currently, the manual is rendered with the timezone of the user running
the rendering. This timezone can fluctuate, depending on the date, due
to DST (Daylight Saving Time). Currently, the manual is rendered in
either CET or CEST (Central European Time, or its DST variant.)

So, a manual rendered during the summer or the winter would refer to a
non-constant timezone. If the machine and/or user doing the rendering
also changes, there is no guarantee the timezone would still be CET/CEST.
This is not a hard issue, since the user can still deduce the time in
UTC, but is just incoherent.

Just force the timezone to be UTC when doing the rendering of the
manual, so we are not dependent on the machine or user doing the
rendering.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>