Fix gh-pages docs publication

[abelsromero]

Found an issue where enabling 'antora' sub-modules causes commits with git-publish to be empty. For now simply remove antora module, the goal is to get base documents up again since we have had already some users asking. I have not reviewed the docs btw, I want to first fix the infra.

• CI:
  • Set write permissions
  • Update active branches
  • Use GH Actions token
• gh-pages Gradle script
  • Update active branches
  • Inject repo from CI env var
• Update .sdkmanrc to Java 11
• Update URL in readme to point to 'master' folder in gh-pages branch.
• Some documents fixes

[abelsromero]

You can see this changes working in my fork https://github.com/abelsromero/asciidoctor-gradle-plugin/commits/gh-pages/

[abelsromero]

I'll merge this and continue working on docs (end goal, getting antora docs in https://docs.asciidoctor.org/) same time tomorrow if there are no objections.