gitlab: Add issue template for documentation issues

Fixes #17180.

.gitlab/issue_templates/documentation_issue.md

+## Summary
+
+Location of documentation issue: (e.g. the Haddocks of `base`, the GHC user's guide)
+
+Write a brief description of the issue.
+
+
+## Proposed improvements or changes
+
+Feel free to propose changes, either concrete or abstract.
+
+
+## Environment
+
+* GHC version used (if appropriate): 
+
+
+
+/label ~bug
+/label ~documentation
+/label ~"needs triage"
+