Document -Z codegen-backend in the unstable book

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+# `codegen-backend`
+
+The tracking issue for this feature is: [#77933](https://github.com/rust-lang/rust/issues/77933).
+
+------------------------
+
+This feature allows you to specify a path to a dynamic library to use as rustc's
+code generation backend at runtime.
+
+Set the `-Zcodegen-backend=<path>` compiler flag to specify the location of the
+backend. The library must contain a function named `__rustc_codegen_backend`
+with a signature of `fn() -> Box<dyn rustc_codegen_ssa::traits::CodegenBackend>`.
+
+## Example
+```rust
+use rustc_codegen_ssa::traits::CodegenBackend;
+
+struct MyBackend;
+
+impl CodegenBackend for MyBackend {
+   // Implement codegen methods
+}
+
+#[no_mangle]
+pub fn __rustc_codegen_backend() -> Box<dyn CodegenBackend> {
+    Box::new(MyBackend)
+}
+```