Automate setting Antora release attributes as part of release job

When we create a release of Antora we want the release job to:

• set the node-version attribute [Deferred]
• define and manage a version-release (or something like it) attribute for the docs to use in the install/upgrade examples attribute
• set version to major.minor.patch (instead of major.minor) in antora.yml in the tag (but not in the branch from where the docs are built for the site)

We're going to do a little experimenting with this, and all attribute names are just placeholders for the ideas right now.