releases: Define rel-latest macro

Add a macro defining the latest current RELEASE that people should use
for new deployments. This will improve consistency and save time bumping
version numbers in handbook examples, which has been suprisingly error
prone.

Jan Brankamp on the Production Users Call suggested we also define a
rel-previous macro. I do not need that yet, but note it here to inspire
future discussion.

Approved by: dch (re)
Differential Revision: https://reviews.freebsd.org/D54293