make text and code box fixed height and allow scrolling

[wilzbach]

again this is just an idea.
The idea is that both the left text and the right text box have a fixed height, so that the reader can always see the navigation menu at the bottom.
Moreover decoupling the two views allows to scroll in the code box without the text moving with your scrolling.
We could map the common navigation key (key {up,down}, page {up, down}) to currently focussed box and by default to left text box.

[stonemaster]

Actually I already had that in the initial version because I found it to be nice in the golang tour. But it effectively reduced the visual space for the content on all screens. And it had issues on mobile. And some users complained about it. And removing made the dlang-tour not look like a golang clone :-)

I'll leave the issue open for now so if everyone wants that we might think about it again.

[stonemaster]

As a follow up: personally I would prefer this design. I just didn't get it right. Maybe we could give this one another try when Bootstrap #255 has been removed. They only main complaints with the first version was that too much space was "wasted" especially on mobile. So if the footer is kept quite small this should be the way to go. Also a very possible fix for #187 because the user directly knows what to do knows using this visual aid.