Expand 'Using JSON' page into a proper guide for JSON

[parlough]

Page URL

https://dart.dev/guides/json

Page source

https://github.com/dart-lang/site-www/tree/main/src/_guides/json.md

Describe the problem

Right now our JSON page just links to other resources. We should expand on this, perhaps with some similar content to Flutter's JSON and serialization page. This is a next step for a full update of the fetching data page: #4520

Expected fix

Update the JSON page, with further inline documentation about utilities and packages such as dart:convert, package:json_serializable, package:built_value, package:freezed, etc.

Additional context

No response

[MaryaBelanger]

Update / clarifying the conversation in #5605:

One goal is to consolidate Dart-specific JSON serialization content on the existing JSON page.

This likely means moving* Dart-specific content from the JSON and serialization page on flutter.dev to dart.dev, since the topic revolves around Dart features and packages.

This is important because, as Dart's JSON serialization story develops, it's important that the source of truth on the topic is clear and distinct.

*Probably doesn't mean removing the entire page, but at minimum transferring the core concepts. The Flutter page can point to the Dart page for "prerequisites".

@parlough let me know if this comment aligns with your thoughts and feel free to edit it