Add configuration for GitHub issue templates

.github/config.yml

+blank_issues_enabled: false
+
+contact_links:
+  - name: Godot proposals
+    url: https://github.com/godotengine/godot-proposals
+    about: Please submit feature proposals on the Godot proposals repository, not here.
+
+  - name: Godot documentation repository
+    url: https://github.com/godotengine/godot-docs
+    about: Please report issues with documentation on the Godot documentation repository, not here.
+
+  - name: Godot community channels
+    url: https://godotengine.org/community
+    about: Please ask for technical support on one of the other community channels, not here.