Doc: Setting navigation variables.

-For Qt 5.2, a navigation bar is implemented:
 b11cb96f282fb1037ad31fbafe279c9a8a8dfeda (qtbase)
-The navigation variables point to pages displayed on the
 navigation bar.

This commit follows the pattern in this commit:
200cad26f258a998d3dc8089445d2c6ae9639ab5 (qtbase)

Change-Id: I56f61af54c782f3167050b0a9d0699cd86936d52
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>