xdge: enable MDX on lib/xdge.mli, fix broken doc example
Run mdx on lib/xdge.mli so the {[ ... ]} odoc block now type-checks.
The example used `env#fs` (object-method shorthand for an
imaginary env), which only works inside Eio_main.run. Wrapped in
`let run () =` and now uses `Eio.Stdenv.fs env`, then prints the
resolved config dir via Fmt.pr so the value is consumed rather than
elided.
Also removed a redundant @see-with-section-heading combo that was
blocking the odoc parse: the {2 Related Libraries} heading sat inside
the @see block, which odoc rejects ("section heading not allowed in
@see"). The same URL was already linked from the introduction's
{{:...} ...} link, so the @see was duplicate.