Migrate "BPMN 2.0" Getting Started Guide

The BPMN Getting Started guide is being replaced by the (new) Quickstart guide.

AT:

• All Screenshots are up to date
• Ensure latest versions of java/javascript external task worker dependencies

RESOURCES:

• Project repo: https://github.com/camunda/camunda-get-started-quickstart
• Static docs repo (push your changes on the 7.9-release branch): https://github.com/camunda/camunda-docs-static/tree/7.9-release
• Adjustment guide