Add details on why built-in COM is not trim safe

[LakshanF]

We’re using a aka.ms link, https://aka.ms/dotnet-illink/com, to point developers to know more about dangers with built-in COM trimming. It currently points to the main doc trimming page. That page has a brief description of built-in COM being incompatible with trimming but it will be good to have a separate page to address this.

The interop overview page, seems like a good candidate to add a new section around trimming.