Add guidance for threading model in native modules

[harinikmsft]

Request from Office/Calendar team: We need the ability to specify which thread a native module should run on. I believe this is possible, or will be soon, but we need some migration document to help us here. Background: We need some of our native modules to run on specific threads. In one case, this needs to be the UI thread, but in other cases, we need to run on a specific dedicated background thread.

[harinikmsft]

For reference, the legacy docs had guidance like this - Threading Model

[chrisglein]

Should document what we currently have, knowing that with turbo modules there will be changes here.