Add documentation for directory "proof" with pointers to existing web pages.

git-svn-id: http://root.cern.ch/svn/root/trunk@17876 27541ba8-7e3a-0410-8455-c3a389f83636

proof/doc/index.txt

+This directory contains the PROOF classes.
+BEGIN_HTML
+ See:
+<ul>
+<li><a href="http://root.cern.ch/twiki/bin/view/ROOT/PROOF">The PROOF system
+    </a></li>
+</ul>
+END_HTML