Link to plaintext for "show source" links (#137131)

Doc/tools/templates/customsourcelink.html

@@ -4,7 +4,7 @@
     <ul class="this-page-menu">
       <li><a href="{{ pathto('bugs') }}">{% trans %}Report a bug{% endtrans %}</a></li>
       <li>
-        <a href="https://github.com/python/cpython/blob/main/Doc/{{ sourcename|replace('.rst.txt', '.rst') }}"
+        <a href="https://github.com/python/cpython/blob/main/Doc/{{ sourcename|replace('.rst.txt', '.rst') }}?plain=1"
             rel="nofollow">{{ _('Show source') }}
         </a>
       </li>