docs: Set English as language for Sphinx (#1875)

* Set 'en' - English as the language for the docs. In Sphinx v5.0.0+ setting
a language is required to avoid:
> WARNING: Invalid configuration value found: 'language = None'

docs/conf.py

@@ -122,7 +122,7 @@ def setup(app):
 #
 # 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: