Add example for exposing Rust types to GDScript, using derive macros and/or wrapper classes

This question is asked pretty frequently, so it's good to have something to point users to.

• Exposing value types through FromVariant and ToVariant.
• Exposing operations on those types, e.g. Eq, through helper methods on a auto-load script.
• Exposing opaque types / types with behaviors through wrapper classes.
• Factory auto-load to work around the constructor argument problem.

[Bromeon]

Partially documented in https://godot-rust.github.io/book/rust-binding.html.
The points mentioned above (especially using a wrapper or auto-load script) still need documentation and/or an example.

[chitoyuu]

Autoload singletons now seem to be documented here: https://godot-rust.github.io/book/gdnative/faq/code.html#can-i-implement-static-methods-in-gdnative

Closing as completed.