[dune] Move to Dune 1.10, use coq.pp directive.

We use the `(coq.pp ...)` dune directive which will produce correct
error messages for `.mlg` files.

Unfortunately we cannot yet use the automatic opam generation features
of Dune 1.10, as this does require a fully native Dune build.

Dune 1.6-1.10 has quite a few other improvements that could be used by
Coq, for example for promote modes.

I have fixed a couple of documentation issues. `Drop` and `ocamldebug`
have been tested in this version.

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