Mono AOT documentation

It'd be nice to have a location to document Mono-specific information and AOT-related topics more generally. The motivation for this issue is this: https://github.com/dotnet/runtime/issues/49805; the Mono runtime currently contains an error message that documents a Mono AOT limitation under a Xamarin iOS category.

I'm not sure where to put this documentation right now. A quick `grep -HRni aot **/*.md` in this repo has 20 hits, with no clear overarching pattern or category.