ocaml-collision: enable MDX on lib/collision.mli
Run mdx on lib/collision.mli so the {[ ... ]} odoc block now
type-checks. The example used a free `cdm`, `let ... in` chains, and
called `Collision.risk_level` (which returns a `risk` variant) where
`%s` expected a string. Wrapped in `let report (cdm : Cdm.t) = ...`
and switched to `Format.printf "%a"` with `Collision.pp_risk` so the
risk classifier renders correctly.