Link to challenges and external resources at beginning of learning materials page

[jonicious]

At Hamburg workshops, we noticed that many people do not scroll through all the learning materials and therefore think, they are only relevant for beginners although they offer great challenges and links to useful external resources. I do not want to change the order of the chapters since this is an event focussed on beginners. Mentioning at the beginning of the learning materials that there are challenges and links to external resource down below would help to make this event more interesting for experienced attendees.

[fredericmarx]

I think it would generally be a good idea to have a more visible chapter navigation, maybe as a sidebar.

A lot of tutorial sites do this; the React Documentation also offers some kind of categorisation into Quick Start and Advanced Guides (see screenshot)