Make the issue tracker template more explicit

Author: pradyunsg

.github/ISSUE_TEMPLATE/issue.yml

@@ -26,6 +26,11 @@ body:
       description: A clear and concise description of what should've happened.
     validations:
       required: true
+  - type: checkboxes
+    attributes:
+      options:
+        - label: I was asked to open an issue in a [discussion](https://github.com/pradyunsg/furo/discussions) by a maintainer.
+          required: true
   - type: checkboxes
     attributes:
       label: Code of Conduct