DOC Add Twitter in Community Section
Adding a Twitter link to the Community section on the homepage of the Scikit-learn website (https://scikit-learn.org/stable/) as mentioned in issue
<li><strong>Questions?</strong> See <a href="faq.html">FAQ</a> and <a href="https://stackoverflow.com/questions/tagged/scikit-learn">stackoverflow</a></li>
<li><strong>Mailing list:</strong> <a href="https://mail.python.org/mailman/listinfo/scikit-learn">scikit-learn@python.org</a></li>
<li><strong>Gitter:</strong> <a href="https://gitter.im/scikit-learn/scikit-learn">gitter.im/scikit-learn</a></li>
+ <li><strong>Twitter:</strong> <a href="https://twitter.com/scikit_learn">@scikit_learn</a></li>
<li>Communication on all channels should respect <a href="https://www.python.org/psf/conduct/">PSF's code of conduct.</a></li>
</ul>
