Add link to `godot-proposals` in the issue template

To reduce confusion over where to propose enhancements to the engine
itself

.github/ISSUE_TEMPLATE/config.yml

@@ -5,6 +5,10 @@ contact_links:
     url: https://godotengine.org/community
     about: Please ask for technical support on one of the other community channels, not here.
 
+  - name: Godot proposals
+    url: https://github.com/godotengine/godot-proposals
+    about: Please submit engine feature proposals on the Godot proposals repository, not here.
+
   - name: Main Godot repository
     url: https://github.com/godotengine/godot
     about: Report engine bugs on the main Godot repository