Describe the pattern of using `.serialized` on a containing suite type to ensure all nested suite types are serialized

[stmontgomery]

Description

We should document the pattern for using .serialized discussed in the comments on https://github.com/swiftlang/swift-testing/issues/683#issuecomment-2344972238.

I think this would be a natural addition to our Parallelization.md DocC article.

[M7md-Ebrahim]

Looking forward to making my first contribution here! Please assign me to this issue

[grynspan]

Thanks for your interest! We have some colleagues who were planning to write this documentation already. If there are specific things you think need to be discussed/covered, please let us know here!

[grynspan]

Unless @iamleeg you would like to work together with @M7md-Ebrahim on it?

[M7md-Ebrahim]

This project aligns with my interests, and I’d love to contribute! I’m also planning to participate in GSoC, so I’m eager to learn, contribute, and work alongside the team @grynspan