Blazor JS intorop serialization section

[guardrex]

Fixes #24646

Thanks @pm100! 🚀

Notes ...

• I prefer not to demo a JSON converter in code given that the cross-linked doc for JSON converters looks good and includes such examples, including the factory approach.
• I'm interpreting remarks from PU discussions, so Tanay please check my remarks carefully ... I might be making up my own .NET or Blazor features here! 🙈😆
• Don't sweat removal of the point-in-time remark+cross-link on DateOnly/TimeOnly for 7.0. Removal of that remark is tracked, so the task won't get lost.
• After we sort out the technical remarks and improve the guidance, I'll create the 5.0 and 3.1 versions of this at the end right before merging the PR.
• ❓ ... The main points from pm100's issue are covered by the PR (and including the extra bits of info from my research that I mentioned on 24646), but are there any other subjects, concerns, or gotchas 😈 that we should address generally in the overview here for serialization?

🇺🇦