Add GitHub Copilot use case to serialize JSON

[anandmeg]

Incorporate GitHub Copilot for VS use case

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Internal previews

📄 File	🔗 Preview link
docs/standard/serialization/system-text-json/how-to.md	docs/standard/serialization/system-text-json/how-to

[anandmeg]

#assign-reviewer:@mijacobs

[anandmeg]

@gewarren, the PR is ready for your review.

[mijacobs]

Could we add an additional set of instructions to the Copilot section? Right now it shows a prompt that defines the types inline. How about a prompt that takes an existing class? Something like "Generate code to serialize the <> class defined in this project to JSON."