doc: Allow resizing of side-nav

Currently it is not possible to resize the side-nav of the RIOT doxygen website in firefox.
This is caused by the window resize_handler being called even for resizing inside the window.
The resize_handler resets the side-nav to a cached size preventing resizing. This commit fixes
this by checking if the window was actually resized.

doc/doxygen/src/js/riot-doxy.js

@@ -46,6 +46,8 @@ function resize_content(sidenav)
 
 function resize_handler()
 {
+    if ($(window).width() == window_before) {return;}
+
     var sidenav = $("#side-nav");
     if ($(window).width() < 750) {
         var toc = $(".toc");