Pass --open to odoc for `mld` files

[favonia]

It seems helpful to pass the open argument to odoc compile when writing tutorials using mld files to open modules in addition to Stdlib, and I wonder if it makes sense to extend the documentation stanza to include a new option open. Something like this in the dune file:

(documentation (open MyModule1.Child1 MyModule2.Child2))

This means odoc compile for the mld files in the same directory will be run with --open MyModule1.Child1 --open MyModule2.Child2.

[rgrinberg]

@Julow @jonludlam what do you think?

[Julow]

This would be very useful but would prevent other drivers for Odoc to do their job (Odig, v3.ocaml.org). The original mld files are installed, not the compiled form, so other drivers would have to guess what is open.

An alternative is to add an open statement to Odoc's syntax.

[emillon]

If I understand correctly, we won't fix that in dune but users interested in this can use the (not existing yet) feature in odoc?

[Julow]

An open statement in mld files is a solution (https://github.com/ocaml/odoc/issues/856) but there's an other solution that can be implemented in Dune: Build and install .odoc files (https://github.com/ocaml/dune/issues/7514)