Revert "main website becomes RTD, main website link is pointless"

Instead, we put the readthedocs.org site as main website.

This reverts commit 8fe56f001cfeeb027162b38da14cfec9f6038754.

docs/_themes/local/sidebarusefullinks.html

@@ -3,6 +3,7 @@
 
 <h3>Useful Links</h3>
 <ul>
+  <li><a href="https://borgbackup.readthedocs.org/">Main Web Site</a></li>
   <li><a href="https://github.com/borgbackup/borg/releases">Releases</a></li>
   <li><a href="https://pypi.python.org/pypi/borgbackup">PyPI packages</a></li>
   <li><a href="https://github.com/borgbackup/borg/blob/master/CHANGES.rst">Current ChangeLog</a></li>