Document how to create add-ins (and similar scenarios)

@RickStrahl is asking for guidance on creating add-ins: https://twitter.com/RickStrahl/status/1551342858774319104. We don't have a doc on that, AFAIK.

I believe the scenario is this:

- Addins can have any dependency.
- Dependencies that are already provided by the app should be knocked out.

We don't have a good way of doing this today. That's what https://github.com/dotnet/runtime/issues/71282 is hoping to enable. The question is whether we have any facility for this today. It sounds like #30341 delivers at least part of that.

Related to
- #30341
- #30340
- https://github.com/dotnet/msbuild/issues/3996
- https://github.com/dotnet/runtime/issues/71282
- https://github.com/NuGet/Home/pull/11708
- https://docs.microsoft.com/visualstudio/msbuild/tutorial-custom-task-code-generation?view=vs-2022#bundle-dependencies-into-the-package

@vitek-karas @rainersigwald @JonDouglas @baronfel @mhutch