Add Q&A template for question proposals

.gitlab/issue_templates/question_proposal_template.md

+## Question 
+[insert question]]
+## Source
+[insert question source]]
+
+## Audience & Location - which FAQ to use?
+- [ ] visitor - on website
+- [ ] user - in user documentation
+- [ ] contributor - in devel documentation
+
+## Answer draft
+[insert answer draft here]
+
+## Validation workflow
+- [ ] identify question
+- [ ] identify source
+- [ ] verify duplication in faq.md
+- [ ] draft answer
+- [ ] get feedback from team
+- [ ] validate by Morane / Benoît / Roberto (depends on scope)