Move documentation to ocamldoc

[mseri]

Since the new odoc, which should be out very soon, supports latex on html output, has extensible output formats (already supports also manpages for example), and does a rather nice job with module aliases and imports (see the recent ICFP talk for details and https://docs.mirage.io/ to see what it looks like: the heavily functorized core documentation shows immediately the improvement), I think it is time to update our mli to use ocamldoc syntax and odoc (master branch) to generate the documentation bundle with cross-linking.