doc: build `biohtml.cmxs`

On some installations `ocamlfind ocamldoc` is configured to use
`ocamldoc.opt`, even if given a `plugin.cmo` file `ocamldoc.opt` will
look for the `plugin.cmxs`, so we just need to build it when the
`$(NATIVE_ENABLED)` veriable is *true*.

This fixes the issue #115.
doc.intro: remove links to `Biocaml_stream/able`