Razor Sdk 3.0 migration docs

[pranavkm]

Things to note in the migration docs

• What do you replace Razor.Design with (ties in to Could not AOT the assembly build failure with version 2.0.1 in Xamarin.iOS projects #2676)?

[pranavkm]

Razor.Design is gone and our migration guides already talk about it. There's some separate additions to the RazorSDK that I'll independently tackle.