mkdocs: Add missing cross-referencing to channels and protobuf

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

mkdocs.yml

@@ -119,6 +119,8 @@ plugins:
           import:
             # See https://mkdocstrings.github.io/python/usage/#import for details
             - https://docs.python.org/3/objects.inv
+            - https://frequenz-floss.github.io/frequenz-channels-python/v1.0-pre/objects.inv
+            - https://googleapis.dev/python/protobuf/latest/objects.inv
             - https://grpc.github.io/grpc/python/objects.inv
             - https://typing-extensions.readthedocs.io/en/stable/objects.inv
   # Note this plugin must be loaded after mkdocstrings to be able to use macros