[gitlab] draft MR template

.gitlab/merge_request_templates/merge-request.md

+<!-- Please use the syntax frama-c/frama-c, else it might close unrelated
+     public issues
+-->
+Close frama-c/frama-c#NNNN
+
+### Description
+
+Your description here
+
+### Companion MRs
+
+No external plug-in impacted
+
+<!--
+- Plugin: link to MR
+-->
+
+### Tasks
+
+- [ ] API documentation is up-to-date, or no need to update
+- [ ] Manuals are up-to-date (and the CI manuals target has been run), or no
+      need to update (including ACSL manual)
+- [ ] Opam dependencies versions are up-to-date (and CI Opam targets have been
+      run), or no changes
+
+### Proposed Changelog Entry
+
+Your entry here