Doc: Use a macro for generating the index page title

Define a macro for generating the index page title with the correct
Qt version (major.minor).

This way, the version is automatically updated on the landing page,
based on .qmake.conf.

Change-Id: I44fcc76bfe4ac899a4d3edb334892b088e021d56
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>

doc/config/qtdoc.qdocconf

@@ -119,3 +119,5 @@ qhp.QtDoc.extraFiles         += images/qt-codesample.png
 
 # Add an .html file with sidebar content, used in the online style
 HTML.stylesheets             += style/qt5-sidebar.html
+
+macro.indextitle              = "\\title Qt $QT_VER"

doc/src/index.qdoc

@@ -26,13 +26,13 @@
 ****************************************************************************/
 
 /*!
+\indextitle
 \page index.html
 \keyword Qt Reference Documentation
 
 \div {class="landing"}
     \div {class="col-1"}
             \div {class="heading"}
-                \title Qt 5.7
             \enddiv
 
                 Qt is a full development framework with tools designed to streamline