Doc: add green icons for next and previous topics

For the online style, now that the text is removed

Change-Id: I13b7da504abda0b395e25dbf6e6ec4bcf03f4ec8
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>

doc/config/qt-html-online-styles.qdocconf

@@ -25,6 +25,8 @@ extraimages.HTML        = qt-logo.png \
                           bg_l_blank.png \
                           bg_ll_blank.png \
                           bg_ul_blank.png \
+                          btn_next_green.png \
+                          btn_prev_green.png \
                           header_bg.png \
                           bg_r.png \
                           box_bg.png \