Fix Sphinx 5 warning by setting docs language

Introduced in Sphinx 5.x:
sphinx-doc/sphinx#10062

We need to define a language in our conf.py

docs/source/conf.py

@@ -64,7 +64,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = "en"
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used: