[html] propagate rel attribute from reference nodes

See https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel,
this allows third-party extensions to set this attribute on the node and have it propagate to the output HTML,
for example executablebooks/MyST-Parser#857

sphinx/writers/html5.py

@@ -325,6 +325,8 @@ def visit_reference(self, node: Element) -> None:
             atts['title'] = node['reftitle']
         if 'target' in node:
             atts['target'] = node['target']
+        if 'rel' in node:
+            atts['rel'] = node['rel']
         self.body.append(self.starttag(node, 'a', '', **atts))
 
         if node.get('secnumber'):