Make why3doc syntax less error-prone

Why3doc offers [...] for embedding code inside comments, but people tend to misuse it. I have already fixed a lot of instances of this issue. I even tried to improve the lexer in 1827453a to workaround the issue. But people keep misusing it. As a consequence, several examples cannot be uploaded to the gallery due to how broken they look.

So, I now propose a much drastic change: forsaking the [...] syntax in why3doc and switching to a Markdown-like syntax: `...`. We could even allow the ```...``` syntax along the way.