Docs: big guide update

- New quick start guide
- New upgrade guide
- New section about providing a product image during integration
- Restructured the docs, so order confirmation is now part of the
integration guide
- Renamed “Integration” to “Product Page Integration” and “Order
Confirmation” to “Order Confirmation Integration” for clarification
- Removed all gists and included the code directly in the docs
- Updated the sitemap
- Fixed links, old code and minor issues

@jtsoi, @zyniq82, @Ekengren have a look, if you want. I know it's tl;dr but a quick scan through might help, as well …