System.Text.Json - Documentation revisions to lower entry point

Please consider revisiting these demos/docs for a more consistent/clear theme. Particularly the relationship between:
- JsonElement (JsonObject, JsonArray, etc...)
- JsonNode
- JsonDocument

Pros/Cons of each. Right now it just seems like a random collection of static methods.

Avoid examples like this without justification (or at least unambiguous variable names):
JsonNode document = JsonNode.Parse(jsonString)!;

This does not exclusively apply to this page, but rather to the entire System.Text.Json namespace documentation.

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* ID: 73a13d38-4904-c068-c9eb-e43176ce5338
* Version Independent ID: c5dfa4d2-c77a-e752-dae7-0bfae5756b76
* Content: [How to use a JSON document, Utf8JsonReader, and Utf8JsonWriter in System.Text.Json](https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-use-dom-utf8jsonreader-utf8jsonwriter?pivots=dotnet-6-0)
* Content Source: [docs/standard/serialization/system-text-json-use-dom-utf8jsonreader-utf8jsonwriter.md](https://github.com/dotnet/docs/blob/main/docs/standard/serialization/system-text-json-use-dom-utf8jsonreader-utf8jsonwriter.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @gewarren
* Microsoft Alias: **gewarren**