Add documentation of enum types

[feois]

Your Godot version: v4.0.2

Issue description: There doesn't seem to be any documentation of enum types' methods (keys(), values(), etc.). I don't even know that enums have methods until I google it.

URL to the documentation page (if already existing):

[Piralein]

For reference:
GDScript Basics points out If you pass a name to the enum, it will put all the keys inside a constant dictionary of that name., but it isn't directly mentioned.

[YuriSizov]

Indeed, enums don't have any methods because they don't really exist as their own entity. I think we can simply add a link to the dictionary documentation with that line, so it's clearer you can use those in this case. Can also add a code sample to illustrate the usage.