doc: howto: interfacing with non-ocaml code

[emillon]

          > I am not sure splitting the document into multiple pages is an improvement since I liked that the topic of dealing with foreign code was fairly contained on one page, but will defer to your judgement on that.

Thanks for the feedback, there's a balance to find here:

• on the one hand, the monolithic page was the only place where foreign_stubs (and friends) were documented, and they contain unrelated documentation (if you're looking for the syntax of (foreign_stubs), it's not likely you're interested in knowing about the history of use_standard_c_and_cxx_flags
• on the other hand, the "here are all your options regarding foreign code in Dune" aspect is now lost.

What I suggest is creating an issue about creating the following doc: an how-to guide which would be "how to use non-ocaml code in my project", with sections corresponding to foreign_stubs (you're in control of this C code) and foreign_archives with optionally foreign_library (this is already built somewhere / there's another build system in there). How does that sound?

Originally posted by @emillon in #10718 (comment)