Online HTML docs

[Armael]

It would be nice to put the ocamldoc-generated docs of the API online. For that we need a make doc target, and I don't know how to do that with jbuilder.

[rgrinberg]

This shouldn't be very hard: http://jbuilder.readthedocs.io/en/latest/api-doc.html#api-documentation

But I don't yet have odoc installed.

[avsm]

if #38 is merged and dune-release is used, then online docs should appear now.

[psafont]

Now that doc generation has been fixed on the latest commit, docs should appear once the next version in https://ocaml.org/p/mustache/latest/doc/index.html

[psafont]

I can see that the docs are published as expected