Docs: Use sphinx-notfound-page to show a nicer 404 page (#111084)

Author: hugovk

Doc/conf.py

@@ -24,7 +24,13 @@
     'sphinx.ext.doctest',
 ]
 
-# Skip if downstream redistributors haven't installed it
+# Skip if downstream redistributors haven't installed them
+try:
+    import notfound.extension
+except ImportError:
+    pass
+else:
+    extensions.append('notfound.extension')
 try:
     import sphinxext.opengraph
 except ImportError:

Doc/requirements.txt

@@ -13,6 +13,7 @@ blurb
 
 sphinx-autobuild
 sphinxext-opengraph==0.7.5
+sphinx-notfound-page==1.0.0
 
 # The theme used by the documentation is stored separately, so we need
 # to install that as well.