provide option to show documentation

[GitMensch]

Maybe it is enough to provide that with a hover, maybe there needs to be a context menu entry for that.
In the easiest implementation it may open the documentation specific to the chosen statement as webview showing local files (content from the programmer's guide).

As some people may not like that option it may should be kept out for a separate extension (which then may have a hard dependency on OCamlPro.SuperBOL).