docs(unstable): Added -Zbuild-dir-new-layout to unstable doc

Author: ranger-ross

src/doc/src/reference/unstable.md

@@ -80,6 +80,7 @@ Each new feature described below should explain how to use it.
     * [feature-unification](#feature-unification) --- Enable new feature unification modes in workspaces
 * Output behavior
     * [artifact-dir](#artifact-dir) --- Adds a directory where artifacts are copied to.
+    * [build-dir-new-layout](#build-dir-new-layout) --- Enables the new build-dir filesystem layout
     * [Different binary name](#different-binary-name) --- Assign a name to the built binary that is separate from the crate name.
     * [root-dir](#root-dir) --- Controls the root directory relative to which paths are printed
 * Compile behavior
@@ -1939,6 +1940,14 @@ The `-Zbuild-analysis` feature records and persists detailed build metrics
 enabled = true
 ```
 
+## build-dir-new-layout
+
+* Tracking Issue: [#15010](https://github.com/rust-lang/cargo/issues/15010)
+
+Enables the new build-dir filesystem layout.
+This layout change unblocks work towards caching and locking improvements.
+
+
 # Stabilized and removed features
 
 ## Compile progress