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

git-svn-id: http://root.cern.ch/svn/root/trunk@17860 27541ba8-7e3a-0410-8455-c3a389f83636

tree/doc/index.txt

+This directory contains all the basic classes for creating/manipulating Trees.
+BEGIN_HTML
+ Several documents describing the TTree classes are listed below:
+<ul>
+<li>The TTree data structure is documented in class TTree.</li>
+<li><a href="http://root.cern.ch/root/HowtoWriteTree.html">How to Create and Fill a Tree 
+    </a></li>
+<li><a href="http://root.cern.ch/root/HowtoReadTree.html">How to Read a Tree 
+      </A></li>
+<li><a href="http://root.cern.ch/root/HowtoChain.html">How to use Chains
+      </A></li>
+</ul>
+END_HTML

tree/src/TTree.cxx

-// @(#)root/tree:$Name:  $:$Id: TTree.cxx,v 1.320 2007/02/06 15:30:25 brun Exp $
+// @(#)root/tree:$Name:  $:$Id: TTree.cxx,v 1.321 2007/02/10 10:17:25 brun Exp $
 // Author: Rene Brun   12/01/96
 
 /*************************************************************************
@@ -32,11 +32,6 @@
 //  Making several branches is particularly interesting in the data analysis
 //  phase, when one wants to histogram some attributes of an object (entry)
 //  without reading all the attributes.
-//Begin_Html
-/*
-<img src="gif/ttree_classtree.gif">
-*/
-//End_Html
 //
 //  ==> TTree *tree = new TTree(name, title)
 //     Creates a Tree with name and title.