Add external reference for `MLJBase.deep_properties`

[pat-alt]

          @pasq-cat I have removed the `@ref` in the `MMI.is_same_except` docstring, because the `deep_properties` is part of MLJBase and cannot be accessed by Documenter.jl unless we use [DocumenterInterLinks.jl](https://juliadocs.org/DocumenterInterLinks.jl/stable/). This looks a little more involved than what I currently have time for, so I'll just open a new issue for this. Feel free to explore this, would eventually be nice to use DocumenterInterLinks.jl to connect different documentations of Taija (but also feel free not to 😅 )

Originally posted by @pat-alt in #126 (comment)