Broken link in godot-core in ExtensionLibrary documentation

[Michael Wigard]

There's a broken link here:

/// # Safety
/// By using godot-rust, you accept the safety considerations [as outlined in the book][safety].
/// Please make sure you fully understand the implications.
/// ...
/// [safety]: https://godot-rust.github.io/book/gdext/advanced/safety.html

[Jan Haller]

Duplicate of #675