Merge branch 'feature/blanchard/gitlab/mr-template' into 'master'

[gitlab] MR template

See merge request frama-c/frama-c!4308

.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
+
+<!-- Only if needed. The list of changes that warrant a Changelog entry is not completely fixed, but includes at least
+  - Bug fix on pub/frama-c (use `##nnnn` to refer to the issue)
+  - Modification that is visible to the end-user
+  - API change, notably function deprecation
+
+   See [Changelog](https://git.frama-c.com/frama-c/frama-c/-/blob/master/Changelog) for how to format an entry. 
+-->