asciidoctor-maven-plugin version should be inherited from parent pom

optaplanner-docs defines the asciidoc, asciidoc-pdf and maven plugin versions.
These should all be definied throught the parent pom.

Requirements:

• check if images still show up in the pdf. (compare the page count before and after)
• check the use cases overview table
• check the branch and bound image that is too big (1024 pixels width) to fit on one page and gets scaled down.