vimhelp: Add commands index to quick links as it's useful

Author: ychin

vimhelp/templates/page.html

@@ -80,6 +80,7 @@ <h1>{{project.name}} help files</h1>
 <a href="{{helptxt}}">help overview</a> &middot;
 <a href="gui_mac.txt.html">MacVim reference</a> &middot;
 <a href="quickref.txt.html">quick reference</a> &middot;
+<a href="index.txt.html">commands index</a> &middot;
 <a href="usr_toc.txt.html">user manual toc</a> &middot;
 <a href="{{helptxt}}#reference_toc">reference manual toc</a>
 {% if project.name == "Vim" %}