Display OS / SOGo upgrade tutorials on index page.

html/index.html

@@ -153,7 +153,13 @@
 <li><a href="srs.html">Enable SRS (Sender Rewriting Scheme) support</a></li>
 <li><a href="store.spamassassin.bayes.in.sql.html">Store SpamAssassin bayes in SQL</a></li>
 <li><a href="track.user.last.login.html">Track user last login time</a></li>
+<li><a href="upgrade.debian.8-9.html">Fixes you need after upgrading Debian from 8 to 9</a></li>
+<li><a href="upgrade.debian.9-10.html">Fixes you need after upgrading Debian from 9 to 10</a></li>
+<li><a href="upgrade.dovecot.2.2-2.3.html">Upgrade Dovecot from 2.2.x to 2.3.x</a></li>
 <li><a href="upgrade.netdata.html">How to upgrade netdata</a></li>
+<li><a href="upgrade.sogo.4.to.5.html">Upgrade SOGo from v4 to v5</a></li>
+<li><a href="upgrade.ubuntu.14.04-16.04.html">Fixes you need after upgrading Ubuntu from 14.04 to 16.04</a></li>
+<li><a href="upgrade.ubuntu.18.04-20.04.html">Upgrade Ubuntu from 18.04 to 20.04</a></li>
 <li><a href="use.a.bought.ssl.certificate.html">Use a bought SSL certificate</a></li>
 <li><a href="use.openldap.as.address.book.in.outlook.html">Use OpenLDAP as address book in Microsoft Outlook</a></li>
 <li><a href="user.alias.address.html">Per-user alias address</a></li>