Maintain XHTML compatibility in split genindex (#12386)

Co-authored-by: Adam Turner <[email protected]>

Author: methane

sphinx/themes/basic/genindex-single.html

@@ -30,7 +30,8 @@
 {% set title = _('Index') %}
 {% block body %}
 
-<h1 id="index">{% trans key=key %}Index &ndash; {{ key }}{% endtrans %}</h1>
+{# We use ``&#x2013;`` instead of ``&ndash;`` for XHTML compatibility #}
+<h1 id="index">{% trans key=key %}Index &#x2013; {{ key }}{% endtrans %}</h1>
 
 <table style="width: 100%" class="indextable"><tr>
   {%- for column in entries|slice(2) if column %}