Mention RFC 3339 support

[tdykstra]

From #21264 (comment)

Add a small/non-obtrusive note/link to RFC 3339 here:
https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to#specify-date-format

And here:
https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to

Or cross-link to our DateTime concept doc to increase its visibility/discoverability. We want folks to be able to understand if and how well our DateTime serialization support fits with ISO 8601 and RFC 3339.