docs/userguide: use a consistent date for reproducible builds

docs/userguide: use a consistent date for reproducible builds

By default, when Sphinx generates the man pages, the current date will
be embedded in them. This can be set to a specific date with the
"today" variable. Typically the date embedded in manpages in the
release date.

To achieve this, attempt to use the environment variable, RELEASE_DATE
to set the "today" variable, reverting back to the empty string if not
set. It is up to our build system to properly set this date.

Ticket: #6911
(cherry picked from commit 51bf1c35103261d075a283d103371a2d9a8a76f7)