Explain how to use @export with custom resources

### Your Godot version:
v4.0.3.stable.official [5222a99f5]

### Issue description:
The [page regarding exports](https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_exports.html) is awesome but it doesn't mention whether or how it's possible to use **custom classes or resources** with `@export` (If I understand correctly only custom resources is possible).

A simple example regarding custom resources would help a lot, e.g.:
`potion.gd`:
```
extends Resource
class_name Potion

@export var name:String
@export var effect:String
```

`example.gd` used by `example.tscn`:
```
extends Node2D
const Potion = preload("res://potion.gd")

@export var equipped:Potion
@export var pouch:Array[Potion] = []
```

### URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_exports.html