Akka Http Documentation preview

I imported the base chapters of the spray documentation and set up the overall structure. As this already now is lots of text you may already start to review.

There are still lots of broken links, mostly because directive documentation is not yet ported. I'll try to get as much directive documentation as possible copied over tomorrow.

I just replaced the current `includecode.py` with our version from spray. If I look at the diff it may be a bit bold to do that because it seems our versions diverged in the past years and you may be depending on some of the features we don't have in the spray version.

/cc @sirthias