Infra: Remove "Contents" and source link from 404 (#4185)

Author: hugovk

pep_sphinx_extensions/pep_theme/templates/page.html

@@ -49,12 +49,12 @@ <h1>Python Enhancement Proposals</h1>
         <article>
             {{ body }}
         </article>
-        {%- if not pagename == "numerical" %}
+        {%- if not pagename.startswith(("404", "numerical")) %}
         <nav id="pep-sidebar">
             <h2>Contents</h2>
             {{ toc }}
             <br>
-            {%- if not pagename.startswith(("numerical", "pep-0000", "topic")) %}
+            {%- if not pagename.startswith(("pep-0000", "topic")) %}
             <a id="source" href="https://github.com/python/peps/blob/main/peps/{{pagename}}.rst">Page Source (GitHub)</a>
             {%- endif %}
         </nav>