[3.14] Link to plaintext for "show source" links (pythonGH-137131) (python#137132)

Link to plaintext for "show source" links (pythonGH-137131)
(cherry picked from commit 9cbf46d)

Co-authored-by: ryan-duve <[email protected]>

Author: miss-islington

Doc/tools/templates/customsourcelink.html

@@ -4,7 +4,7 @@ <h3>{{ _('This page') }}</h3>
     <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>