[coqdoc] support for "--external"

When building Coq documentation, it would be pretty cool if the documentation of the project's dependencies could be linked automatically. An example of how such documentation is linked can be found here (taken from this Coq zulip message).

One possible idea is to automatically generate --external flags automatically for documentation. For example, if the theory you want to generate documentation for depends on stdpp, they you want to generate something like --external 'https://plv.mpi-sws.org/coqdoc/stdpp/' stdpp. This could be done by looking for the relevant URL in the stdpp opam file, which could be embedded as a specific metadata field that dune could produce in the opam files it constructs for theories with documentation.

CC @Alizter

Start by reviewing the linked Makefile example and the coqdoc --external behavior described in the issue. Then investigate how Dune constructs opam files and whether dependency documentation URLs can be represented as metadata. Done means defining and implementing a clear way to generate external links for documented dependencies.