doc: add title to index

PR-URL: #62046
Reviewed-By: Claudio Wunder <cwunder@gnome.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: avivkeller

doc/api/index.md

@@ -1,7 +1,4 @@
-<!--
-  NB(chrisdickinson): if you move this file, be sure to update
-  tools/doc/html.mjs to point at the new location.
--->
+# Index
 
 <!--introduced_in=v0.10.0-->