mdx: switch doc-example printers from Format/Printf to Fmt
Repo convention is to use the `fmt` library's Fmt.pr / Fmt.epr / etc.
in code, and the recently added mdx examples drifted to
Format.eprintf / Printf.eprintf out of habit. Brought them back into
line so a reader copy-pasting the example doesn't pick up the
wrong style.
Touches the {[ ... ]} blocks in:
- ocaml-collision/lib/collision.mli
- ocaml-cose/lib/cose.mli
- ocaml-delegation/lib/delegation.mli
- ocaml-gdocs/lib/gdocs.mli (also added fmt to the mdx libraries)
- ocaml-runc/lib/runc.mli (also added fmt to the mdx libraries; \n
format escapes became @. for consistency)
- ocaml-sigstore/lib/sigstore.mli