Adding images of CMMN elements

[a-m-zill]

Problem

On the CMMN overview page (https://docs.operaton.org/docs/documentation/reference/cmmn11/) are images missed

How to fix

When you compare to the original page of camunda (https://docs.camunda.org/manual/7.24/reference/cmmn11/), you see the missing images.

One way can be to activate the CMMN elements in the modeler. See also on the overview page. There is a information how to do this.
Then create the element and make screenshots with good names of them. Add this to the img folder in the cmmn docs docs/documentation/reference/cmmn11/img. After this reference them in the index.md in the folder cmmn11.