Simplifying Opam-doc process

2 changes:

1) add a new compiler, 4.01.0+bin-doc which is the same as 4.00.1+bin-doc except for the OCaml version
2) switching the opam-doc from lpw25/opam-doc to ocamllabs/opam-doc

Check the pull request on ocamllabs/opam-doc first as this has the corresponding changes to the Makefile to add "install"