docs: Add Reasoning Engine reference documentation

PiperOrigin-RevId: 623891416

Author: yeesian

docs/vertexai/services.rst

@@ -49,3 +49,8 @@ Vertex AI SDK
  :members:
  :show-inheritance:
  :inherited-members:
+
+.. automodule:: vertexai.preview.reasoning_engines
+ :members:
+ :show-inheritance:
+ :inherited-members: