doc: Add sphinx 404 page

doc/conf.py

@@ -88,10 +88,14 @@ extensions = [
     "sphinx.ext.extlinks",
 ]
 
+if on_rtd:
+    extensions += [
+        # The custom 404 page is only needed
+        "notfound.extension",
         # The search extension works only on readthedocs
         # See https://readthedocs-sphinx-search.readthedocs.io
-if on_rtd:
-    extensions.append("sphinx_search.extension")
+        "sphinx_search.extension",
+    ]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = [doc_path + '/_templates']

doc/requirements.txt

@@ -1,2 +1,3 @@
 sphinx-rtd-theme~=1.3.0
+sphinx-notfound-page~=1.0.0
 readthedocs-sphinx-search~=0.3.1