Draft: Extend the MR template, note that a general summary is now mandatory

The merge request template has been extended. From now on a general summary for a MR is mandatory.

Description

Creating a new merge request with the help of the default template will now inform the user that it is mandatory to provide a general summary. From now on merge request without one will not be merged anymore (this applies for everyone, no excuses).

In general it is preferred that all the sections in the template are addressed but we expect at least a proper general summary from now on.

Motivation and Context

Most of our merge requests do not provide proper explanation about the changes and their purpose. Providing a general summary makes it easier to understand the context of a MR and it will also help us in the future to list all the changes in a new release.