[infra] Eliminate use of relref in spec and other pages

[chalin]

Eliminate the use of relref in spec pages (as introduced by the adjust-pages.pl script), because the relref shortcode emits an error when the reference cannot be resolved, and this aborts the build, which prevents us from getting at least a partial build of the site.

As mentioned in the Hugo page previously referenced, the behavior of relref is in large part subsumed by link render hooks.

We might also want to eliminate use of relref across all site pages.

Tasks

• Auto-update registry versions (eeaec9310075186778bbabe75532f5c9521ad913) #6023
• ...