Ensure sarif extra is included as part of doc build (#1139)

The doc build nowadays runs via the readthedocs.yaml file. So
the requirements for building those docs need to include sarif
in order to correctly build the sarif formatter doc.

Fixes: #1138

Signed-off-by: Eric Brown <eric_wade_brown@yahoo.com>

.readthedocs.yaml

@@ -14,3 +14,5 @@ python:
     - requirements: doc/requirements.txt
     - method: pip
       path: .
+      extra_requirements:
+        - sarif