[docs] update config to specify language

Otherwise sphinx 5.x fails with the error:

    Warning, treated as error:
    Invalid configuration value found: 'language = None'.

docs/conf.py

@@ -72,7 +72,7 @@
 #
 # 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"
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.