Replace the [...] syntax from why3doc by a Markdown-like syntax (fix issue #132).
why3doc comments in the gallery contains
[i,j[, which breaks the documentation since it opens a code block that is never closed. The proper way to write it is
[\[i,j\, which is not only ugly but also error-prone.
This commit introduces a Markdown-like syntax for embedded code. It uses the backtick as a delimiter:
`[i,j[`. It also supports multi-backticks: you can output
`` `code` ``.