Infrastructure docs

Here is the basis infrastructure for the docs, this PR does contain 2 little docs pages, but those are more likely placeholders. Do you like this infrastructure? If something needs to be changed, ping me. 

The docs are as markdown, so those are available without any kind of webserver. But later on gulp generates `site/docs`, so the docs can be accessed in a browser too. 

Once this is merged, we can start writing ;-)