Docs: Update sphinx-wagtail-theme to 5.0.4 (#7296)

docs/requirements.txt

@@ -1,2 +1,2 @@
 docutils==0.16  # Pinned to work around https://github.com/sphinx-doc/sphinx/issues/9049
-sphinx-wagtail-theme==5.0.3
+sphinx-wagtail-theme==5.0.4

setup.py

@@ -76,7 +76,7 @@ documentation_extras = [
     'sphinxcontrib-spelling>=5.4.0,<6',
     'Sphinx>=1.5.2',
     'sphinx-autobuild>=0.6.0',
-    'sphinx-wagtail-theme==5.0.3',
+    'sphinx-wagtail-theme==5.0.4',
     'recommonmark>=0.7.1',
 ]