ocaml-sdls: enable MDX on eio/otar_eio.mli
Run mdx on eio/otar_eio.mli so the {[ ... ]} odoc block now
type-checks against the real Sdls.Otar / Sdls_eio.Otar_eio APIs.
The example referenced free `fs`, `kek`, `master_material`, `cmd`,
`send`, `log_error`, used unqualified `Otar_eio.of_path`, and
`let ... in` chains that do not parse at the toplevel. Wrapped in
`let run ~kek ~master_material ~cmd ~send = Eio_main.run @@ ...` so
the user supplies the credentials and command, the example shows
both the Ok reply and Error paths via Fmt.epr + Sdls.Otar.pp_error,
and the disk-touching of_path call doesn't execute at mdx test time.