Add link to -Zcodegen-backend unstable Cargo feature

Author: Kobzol

src/doc/src/guide/build-performance.md

@@ -63,7 +63,7 @@ This will change the [`dev` profile](../reference/profiles.md#dev) to use the [C
 
 Trade-offs:
 - ✅ Faster code generation (`cargo build`)
-- ❌ **Requires using nightly Rust and an unstable Cargo feature**
+- ❌ **Requires using nightly Rust and an [unstable Cargo feature][codegen-backend-feature]**
 - ❌ Worse runtime performance of the generated code
   - Speeds up build part of `cargo test`, but might increase its test execution part
 - ❌ Only available for [certain targets](https://github.com/rust-lang/rustc_codegen_cranelift?tab=readme-ov-file#platform-support)
@@ -86,4 +86,4 @@ Trade-offs:
 
 [parallel-frontend-blog]: https://blog.rust-lang.org/2023/11/09/parallel-rustc/
 [parallel-frontend-issue]: https://github.com/rust-lang/rust/issues/113349
-
+[codegen-backend-feature]: ../reference/unstable.md#codegen-backend