MNT update date and links on the index webpage
<ul class="sk-landing-call-list list-unstyled">
<li><strong>On-going development:</strong>
<a href="https://scikit-learn.org/dev/whats_new.html"><strong>What's new</strong> (Changelog)</a>
+ <li><strong>August 2020.</strong> scikit-learn 0.23.2 is available for download (<a href="whats_new/v0.23.html#version-0-23-2">Changelog</a>).
+ </li>
+ <li><strong>May 2020.</strong> scikit-learn 0.23.1 is available for download (<a href="whats_new/v0.23.html#version-0-23-1">Changelog</a>).
+ </li>
<li><strong>May 2020.</strong> scikit-learn 0.23.0 is available for download (<a href="whats_new/v0.23.html#version-0-23-0">Changelog</a>).
</li>
<li><strong>Scikit-learn from 0.23 requires Python 3.6 or greater.</strong>
