Fix for multicolumns user manual menu in devices with 480px or less

[ivanhercaz]

In reference to #11 and #17, I made this fix to change the three columns of the user manual menu to two columns. It should work without !important and any additional media query, but as we seen in #17, maybe could be necessary to make another trick like use !important or made another media query, although this isn't confirmed that work per #20.

Test it in live, @kghbln, and tell me if it works only with this code or if it's necessary to use one of the both tricks I mentioned.

Regards,
Iván

[kghbln]

@ivanhercaz Thanks a lot for this easy as well as smart solution. On a narrow screen the menu is to much aligned to the left so it gets hidden. Thus we need another tweak.

The only thing that really fatigues me is that it just puts the content of the third column to the second one. It will be nice if the content of the second column gets added to the first before, but I am afraid we would need JavaScript for this. Never mind however, I still need to clean out links to make this less apparent.