docs: use default language 'en' (#2538)

Since sphinx-5.0.0 "None" is not recommended anymore.

docs/conf.py

@@ -58,7 +58,7 @@ master_doc = 'index'
 #
 # 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.