Add first commit message to MR template

Francois Eleouetrequested to merge
fe/commit-message-in-mr into main

What does this MR do and why?

As commit message may contain meaningful explanations, it is worth adding it to the MR summary.

Is is easier to delete this content if it is not relevant rather than having to copy/past it from commit.

(commit content is included by default in other project merge requests.)

Merge request reports