Migrate the "DMN" Getting Started Guide

AT:

• All Screenshots are up to date
• It works with 7.12.0
• Code Examples in the docs use 7.12.0 version
• Source Code in solution repository uses 7.12.0 Version
• Adjust the license header

RESOURCES:

• Project repo: https://github.com/camunda/camunda-get-started-dmn
• Static docs repo (push your changes on the 7.12-release branch): https://github.com/camunda/camunda-docs-static/tree/7.12-release
• Adjustment guide