Build documentation as a part of the optaplanner website

The docs website should be a part of the optaplanner website and thus it should build as its module.

Challenges:

• during the release, the Antora playbook must point to the right branch or multiple branches (if we want to make old documentation versions a part of the docs website)
• the antora.yml inside the optaplanner repo needs to contain the correct version on every included branch
• the asciidoc attributes in the antora.yml inside the optaplanner repo need to be updated with every release