feat: add support for optional event warning for in-line annotation

Add warning for openedx-events annotations so developers can include
relevant information about the current event. As for now, these warnings
have been used to indicate that the event at the time of implementation
doesn't have for the event bus.

code_annotations/contrib/config/openedx_events_annotations.yaml

@@ -12,6 +12,7 @@ annotations:
     - ".. event_description:":
     - ".. event_data:":
     - ".. event_key_field:":
+    - ".. event_warning:":
 extensions:
     python:
         - py

code_annotations/contrib/sphinx/extensions/openedx_events.py

@@ -118,6 +118,11 @@ def iter_nodes(self):
                      f" {event['line_number']})"
             )
 
+            if event.get(".. event_warning:") not in (None, "None", "n/a", "N/A"):
+                event_section += nodes.warning(
+                    "", nodes.paragraph("", event[".. event_warning:"]), ids=[f"warning-{event_name}"]
+                )
+
             subject_header += event_section
 
         if domain_header: