Create documentation for multithreaded mode

Multithreaded mode changes how task authors must write tasks, and it can change what a build does. Task authors need to know which APIs are safe and how to migrate. Build owners need to know which behaviours differ from multi-process builds before they turn `-mt` on/off. This epic produces that documentation.

## Steps

- [x] [#11838](https://github.com/dotnet/msbuild/issues/11838) Threat model the multithreaded execution mode
- [x] [#11832](https://github.com/dotnet/msbuild/issues/11832) Publish task author docs and samples for the new task type and node type
- [ ] [#13696](https://github.com/dotnet/msbuild/issues/13696) Document how to migrate tasks that must keep working on older MSBuild [size XS] [Milestone: version 18.13]
- [ ] [#12906](https://github.com/dotnet/msbuild/issues/12906) Document the behavior changes users may see in multithreaded mode [size S] [Milestone: SDK 11.0.200]

## Stakeholders

Task authors inside and outside Microsoft, and build owners deciding whether to turn on `-mt`.

## Definition of done

A task author can find published docs and samples showing how to write a task for multithreaded mode. Build owners have a documented list of behaviour differences between multithreaded and multi-process builds.