The entire page is editable and a live demo. It could probably be higher up, but it is mentioned on the page.
Wow. Ok Thank you. This morning looking at it I was blind to those widgets.
> Try for yourself: You may not have noticed, but you are already in edit mode! Just click anywhere in the text to edit
On my screen (1080p) I have to scroll to see this and it's in plain small body text, doesn't stand out at all.
Additionally to the sibling comments: you can also check out the demo at demo.edtr.io that contains way more plugins. It’s pretty much what we use on Serlo.org.
I thought that until I realized that image that's there IS the actual editor