Linkable headers

[HugoGranstrom]

Similarly to the cheatsheet example in Nimib I think making all the headers linkable is a good idea. That would make it easier to direct people asking questions to specific sections among other things. It would also allow us to have TOCs on every page like in the cheatsheet example if we wanted to. The current implementation needs some work though, optimally the headers themself (or a symbol in front/after of them) should be clickable so one easily can copy the link.

@pietroppeter Do you think this is functionality that (optionally) should be offered by Nimib/NimiBook itself or should we roll our own version here?