DOC: Adding gitlab templates for issues

.gitlab/issue_templates/bugs.md

+## Title of the bug
+
+### Description
+
+Describe what happens and why you think it is a bug
+
+### Steps to reproduce
+
+Describe as precisely as possible how to reproduce the bug. Try to isolate a minimal number of steps. Also describe reproducibility (always, random ...).
+
+### Configuration information
+
+OS, OTB version or tag, information related to build (binaries, superbuild, system libs ...)

.gitlab/issue_templates/documentation_changes.md

+## Title of the documentation change requested
+
+### Target documentation ressources
+
+Can be a combination of Software guide, cookbook, doxygen, blog, applications doc, websites
+
+### Change requested
+
+Describe precisely the changes that are required.

.gitlab/issue_templates/feature_request.md

+## Title of the requested feature
+
+Short summary of the requested feature

.gitlab/issue_templates/request_for_comments.md

+## Title of the resquest for comments
+
+### What changes will be made and why they would make a better Orfeo ToolBox?
+
+#### High level description
+
+#### Risks and benefits
+
+#### Alternatives for implementations
+
+### Who will be developing the proposed changes?
+