Remove overlap of different documentation config files

(From a discussion on GitHub)

There are these files:

• antora.yml
• playbook.yml (and its dev sibling playbook_author.yml)
• _attributes.adoc
• _config/site.yml (in the website repo)

All of which seem to contribute to the configuration of rendered documentation. There relationships are somewhat unclear to me. We should:

• Remove duplications between those files (e.g. why is there "debezium-docker-label" in site.yml?)
• Develop a model for organizing the properties in a non-redundant way and describe this in DOCUMENTATION.md
• _attributes.adoc: the version in "mysql-connector-plugin-download" should be replaced with an attribute reference
• On a tangent: let's also close/merge the Dependabot PRs for debezium.github.io. I believe most/all of them are obsolete due to the recent Awestruct update, but worth double-checking.