add unstable-books doc for offload

Author: ZuseZ4

src/doc/unstable-book/src/compiler-flags/offload.md

@@ -0,0 +1,8 @@
+# `offload`
+
+The tracking issue for this feature is: [#131513](https://github.com/rust-lang/rust/issues/131513).
+
+------------------------
+
+This feature will later allow you to run functions on GPUs. It is work in progress.
+Set the `-Zoffload=Enable` compiler flag to experiment with it.