OxCaml: support for kind abbreviations

[art-w]

This PR adds support for rendering and linking to kind abbreviations (this introduces a new namespace, similar to types, so a lot of mechanical changes are required!).

As we are currently testing with OxCaml-minus31, there's a small inconvenience that one has to write an explicit [@@ocaml.doc "..."] to document kind_ abbreviation as the parser was missing an add_docs_attrs ( https://github.com/oxcaml/oxcaml/blob/5.2.0minus-31/parsing/parser.mly#L4123-L4134 ). The oxcaml parser has since been fixed so we'll be able to clean up the test in the near future :)

Fix #1443

[panglesd]

Odoc references have this quite particular semantic that they can be "forward reference". So for instance:

type t
(** Abstract but can be created with {!v} *)

val v : unit -> t

The reference to v is made before v is defined.

As a result, in the following example:

kind_ to_be_shadowed = value mod portable

module X : sig
  type t : to_be_shadowed

  kind_ to_be_shadowed = value_or_null mod non_null global
end

When clicking on to_be_shadowed associated to the type t, odoc redirects to the second abbrev, value_or_null mod non_null global. I believe the first one, value mod portable, is the one oxcaml would use.

I'll think about what other path you could use. (But tell me if this example is wrong!)