Add back button to `subject_indexes#show`

app/views/subject_indexes/show.html.erb

@@ -24,6 +24,7 @@
       </div>
 
       <div class="col-sm-2 d-flex justify-content-end align-items-center pe-0">
+        <a class="me-3" href="<%= subject_indexes_path %>"> <i class="fas fa-angle-double-left" style="font-size: 20px;color: darkred;" data-bs-toggle="tooltip" data-bs-placement="right" title="Back to index"></i> </a>
         <a class="btn btn-secondary me-3 btn-sm mb-1" href="<%= edit_subject_index_path(@subject_index) %>"> Edit </a>
         
         <% if !@subject_index.has_children? %>