fix: set sphinx language to 'en'

sphinx-quickstart previously generated `language = None` by default. As of Sphinx version 5, language will default to en.

For proof this is happening, see note on https://github.com/sphinx-doc/sphinx/blob/e1bf4dd5d5860a4c3790f41c5f5fe389dc5b4cf9/sphinx/config.py#L167-L174.

For context on why this is happening, see sphinx-doc/sphinx#10474.

docs/conf.py

@@ -125,7 +125,7 @@ def get_version(*file_paths):
 #
 # 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: