Update release procedure to cover required documentation config changes

See this chat conversation:

[There was a] problem with the 1.8.0.Final release where we didn't adjust the prerelease descriptor in the antora.yml from true to false.

This step should be described in the release procedure guide. Looking at this one, Antora is mentioned in two places:

• Update antora.yml and series.yml
• Documentation post-release changes

The scope of these two is not 100% clear to me: does the former apply to both manual and automated releases? And the latter only to manual releases? Or is this step also required for automated releases. This may deserve some re-org/clarification.