Rename .NET Android, .NET iOS to .NET for Android, .NET for iOS

[rachelkang]

Issue description

We need to include "for" in the product names due to marketing and branding guidelines, so we need to update any place in the docs that is not already following this convention (i.e. https://github.com/dotnet/docs-maui/blob/main/docs/what-is-maui.md)

.NET Android --> .NET for Android .NET iOS --> .NET for iOS