Create a two-level sidebar TOC

The sidebar TOC mechanism used in "\installer-framework\doc\config\style\qt5-sidebar.html" supports several levels of entries. In this doc set, two levels would be useful: https://doc-snapshots.qt.io/qtifw-master/index.html

For example:

<div class="sectionlist normallist">
    <div class="heading">
        <h2>Getting Started</h2>
    </div>
    <div class="indexboxcont indexboxbar">
        <ul>
            <li><a href="ifw-getting-started.html">Getting Started</a></li>
            <li><a href="ifw-overview.html">Overview</a></li>
            <li><a href="ifw-installation.html">Installation</a></li>
        </ul>
    </div>
</div>

...