Drools 8 docs : Rule Unit API

Issue Description: <{}This is the problem you are trying to solve. Please use instructions of how to recreate the issue and screenshots to help>{}

• Add Rule Unit API section in "BuildDeployUtilizeAndRun" chapter.
• Use archetype example
• Consider description duplication in docs (Getting Started, Rule Reference Guide, Migration Guide)
• Move API related description from Rule Reference Guide "Rule units in DRL"

Acceptance Criteria: <Use this section to define the definition of done for this issue>

• Users can naturally learn Rule Unit API in the docs.

Out of Scope: <Here you need to fill in what is out of scope for this issue and then create new issues if necessary in the backlog for the work>

• We may postpone final brush-up about all Rule Unit information arrangement.

Additional Information: <Add any additional information here that may be helpful to people reading the ticket>

N/A