Set language in code blocks

[ulrikstrid]

It would be nice if we could tell odoc that a particular code block is either OCaml or Reason. If there is a is a integrated mode where we can switch syntax on the future it would be nice to provide examples in both languages and be able to switch between them.

[aantron]

It seems we need to extend the ocamldoc language for this. If so, we should also begin documenting odoc's language extensions.

[trefis]

The previous version of odoc/octavius had documentation for the new syntax: https://ocaml-doc.github.io/octavius/syntax.html

I don't know if there have been much changes since, but you can probably start from there (here is the mld).

[dbuenzli]

Also related to #303

[github-actions]

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