Add link to verification documentation

Author: di

templates/downloads/release_detail.html

@@ -50,7 +50,7 @@ <h1 class="page-title">Files</h1>
               <th>File Size</th>
               <th>GPG</th>
               {% if release_files|has_sigstore_materials %}
-              <th colspan="2">Sigstore</th>
+              <th colspan="2"><a href="https://www.python.org/download/sigstore/">Sigstore</a></th>
               {% endif %}
             </tr>
           </thead>