Integrate MDoc for API documentation

[jamesmontemagno]

See: https://github.com/xamarin/Caboodle/tree/master/Caboodle

[jamescurran]

I believe that link now wants to be : https://github.com/xamarin/Essentials/tree/master/Xamarin.Essentials

[jfversluis]

Including this is probably still relevant, so keeping this one here so we won't forget

[Cfun1]

@jfversluis is this in progress ?

[jfversluis]

We have a repo setup, which is private

Now deciding on how to generate the docs and get them there as part of our builds/releases