Write a manual

[sonmarcho]

Charon is starting to have quite a few undocumented features. For instance, it is now possible to annotate declarations with the opaque attribute. We should start thinking about writing a reference manual.

[Nadrieril]

Any help here is needed. I'm more than happy to fill holes with the knowledge I have, what's difficult for me is getting started on what needs to be explained and in what order.