doc: silence warnings from openapi sphinx extension

it prints out

skipping non-JSON example generation.

if an API does not contains example while ":example:" is specified for
openapi extension, and if this API is not a GET request, openapi
extention complains.

see also
9dbae9c9a6/sphinxcontrib/openapi/openapi30.py (L191)

Signed-off-by: Kefu Chai <kchai@redhat.com>

doc/conf.py

@@ -1,4 +1,5 @@
 import fileinput
+import logging
 import os
 import shutil
 import sys
@@ -213,6 +214,10 @@ for c in pybinds:
     if pybind not in sys.path:
         sys.path.insert(0, pybind)
 
+# openapi
+openapi_logger = logging.getLogger('sphinxcontrib.openapi.openapi30')
+openapi_logger.setLevel(logging.WARNING)
+
 
 # handles edit-on-github and old version warning display
 def setup(app):