13.2R: RC3 date, link to checksums, RC context

Add the actual date when builds of RC3 began.

Enable the link from
https://www.freebsd.org/releases/13.2R/
to the signatures page.

Add release candidate context to the signatures page. For now, hide
the RELEASE alternative to the line.

Approved by: doc (gbe)
Differential revision: https://reviews.freebsd.org/D39170