fix(docs): Add a warning when using Sphinx 3

Hawkmoth is not yet working with Spinx 3; issue a warning when detecting
a Sphinx 3 environment as the resulting docs will be broken.

Signed-off-by: Rahix <rahix@rahix.de>

Documentation/conf.py

@@ -15,6 +15,14 @@ sys.path.insert(0, os.path.abspath("./"))
 
 logger = sphinx.util.logging.getLogger("card10/conf.py")
 
+# Hawkmoth does not yet support Sphinx 3
+#
+# See https://github.com/jnikula/hawkmoth/issues/17
+if sphinx.version_info[0] > 2:
+    logger.warning(
+        "Sphinx versions >= 3 are not yet working properly with hawkmoth.  Documentation of C items will probably be broken ..."
+    )
+
 
 # -- Project information -----------------------------------------------------