Add concept: Maybe

[MatthijsBlom]

This is my WIP w.r.t. #1103.

[MatthijsBlom]

@kytrinyx @ErikSchierboom Before I continue with the last bits of Maybe and then Either and tuples: what do you think of this style and depth?

[kytrinyx]

@MatthijsBlom Thank you so much for taking the time to think about this.

I think there's a lot of good stuff in here, but I'm unable to say anything concrete about it until we know what the exercise looks like.

There was a conversation in the Slack a couple of weeks ago which might have gotten lost, as there were about a dozen conversations happening in different places all at once.

Peter Wadsworth asked:

What process should be followed for developing concepts?

And my response was that there are four steps.

1. Identifying the right concepts to teach at the current point in the concept tree
2. Figuring out the piece of code that will be the basis for the exercise
3. Fleshing out the code part of the exercise
4. Working out the documentation (exercise introduction, instructions, concept introduction)

It is really important that we complete each one before moving to the next.

At the moment we've identified Maybe (and Either and tuples) as concepts we want to tackle (step 1).

In order to avoid rework and wasted effort we need to spend some time on step 2 for all of these concepts: looking at a handful of different code examples that use the concept in simple scenarios. It's useful to have several examples rather than just one, so that we can narrow in on what feels like the right size, shape, and conceptual complexity of the target exercise.

I hope this makes sense. Once we have the code example nailed down it will be much easier to talk about this PR in depth!