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

git-svn-id: http://root.cern.ch/svn/root/trunk@17863 27541ba8-7e3a-0410-8455-c3a389f83636

gui/doc/index.txt

+This directory contains all the low level GUI classes.
+BEGIN_HTML
+ Several documents describing the I/O classes are listed below:
+<ul>
+<li><a href="http://root.cern.ch/root/HowtoGUI.html">How to use the GUI classes
+    </a></li>
+<li><a href="http://root.cern.ch/root/HowtoSignalslot.html">How to use the Signal/Slot communication mechanism
+      </A></li>
+<li><a href="http://root.cern.ch/root/HowtoEdit.html">How to use the Graphics Editor
+      </A></li>
+<li><a href="http://root.cern.ch/root/HowtoGuiBuilder.html">How to use the GUI Builder
+      </A></li>
+<li><a href="http://root.cern.ch/root/HowtoStyle.html">How to Create or Modify a style
+      </A></li>
+</ul>
+END_HTML