Create spec/doc for JSON serialization

[krwq]

Currently JSON serializer has 3 code paths which are distinct implementations:

• regular path
• fast path
• code gen

we're finding more and more issues related to divergence in behavior.

I think it would be useful to spec out the expected behavior and define how it should work. We should also research other similar specs and existing implementations before finalizing it.

Benefits:

• source of truth for behavior
• possible interop with different languages
• easier to find potential gaps and improvements
• good place to add security considerations and notes to prevent us from accidentally going into the path of BinaryFormatter

[layomia]

Dup of dotnet/docs#26040. @tdykstra and I have started working on this.