fix(confluent-kafka): fix malformed RST in instrumentation docs

Add missing `:end-before: References` directive and `API` section
header to confluent_kafka docs, matching the pattern used by
aiokafka and kafka_python instrumentation docs.

Without `:end-before:`, the raw References URLs were rendered
inline in the docs page. The missing API header left the
automodule section unlabeled.

Fixes #3061

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mvanhorn

docs/instrumentation/confluent_kafka/confluent_kafka.rst

@@ -1,7 +1,10 @@
 .. include:: ../../../instrumentation/opentelemetry-instrumentation-confluent-kafka/README.rst
+    :end-before: References
+
+API
+---
 
 .. automodule:: opentelemetry.instrumentation.confluent_kafka
     :members:
     :undoc-members:
     :show-inheritance:
-    :noindex: