Clarify nuget/mono isn't supported and never was on Unixy platforms.

[JonDouglas]

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[zivkan]

There are 35 files in this docs repo that mention mono. This is a good first step, but I think we need to change wording on other pages too, for example, the page on installing/obtaining nuget. For better or worse, about half the mono search results look to be release notes, and I don't think we need to make any changes there, so I think the effort required is smaller than the 35 files with results makes it sound to be.