Added `Next`-Link in setup and basic propel docs

Now you don't have to scroll back to top every time
when you finished reading the page.
Merge pull request #328 from mpscholten/next-link

Added `Next`-Link in setup and basic propel docs
added about-us, adjusted the layout a bit and removed the loud green color at the startpage, made documentation 100% width for better readability, Fixed fork-and-edit link.