Create an example to show how to customise headings.

[01mf02]

This should address #2351.

[laurmaedje]

I am not entirely comfortable with this example in the docs. The box(width: 35pt, ..) thing is relatively hacky (I know that I proposed it in the linked issue). I understand that it would be useful to have more examples in the docs, but the docs also have normative character, so I would prefer to only add idiomatic examples there. In particular, the box solution ensured that the heading's title is always at the same position (independently of the numbering), which is not exactly what the description here reflects.

I am also not sure in how far the top section of each element should accumulate various different examples for the element. It will shift the synopsis down quite a lot. I think we need to figure something out for adding more cookbook-style examples.

[laurmaedje]

I'll close this for now, but we can reopen should more discussion come to some conclusion.

[01mf02]

OK! Thanks for taking the time to explain.