Make to_json() easer to find

**Your Godot version:**

3.5.1 Stable Official (through Steam)

**Issue description:**
the documentation for the JSON helper class (or the JSONParseResult class) should include a reference to the to_json() function or to the serialization section of the saving_games page, since developers are likely to find this page while looking for how to turn objects into JSON.

An example line - "If you're looking for how to create JSON from existing objects, see serialization"

**URL to the documentation page (if already existing):**

serialization section of saving_games: https://docs.godotengine.org/en/latest/tutorials/io/saving_games.html?highlight=to_json#serializing
JSON Parse Result: https://docs.godotengine.org/en/stable/classes/class_jsonparseresult.html
JSON: https://docs.godotengine.org/en/stable/classes/class_json.html