Improve markdown

[qdii]

Extend the markdown syntax, maybe like this:
• Add article titles slugification and allow [[page name]]
as a short for [page name](page name)
• [[page name/eng]] to do link to pages from other languages
• Disallow parenthesis or other characters that could mess up with
the link syntax in article names.

[allan-simon]

I don't know how much of this ticket would be covered by #93 ?