add sort_unstable to unstable book

cc #40585

Author: steveklabnik

src/doc/unstable-book/src/SUMMARY.md

@@ -75,6 +75,7 @@
 - [simd](simd.md)
 - [simd_ffi](simd-ffi.md)
 - [slice_patterns](slice-patterns.md)
+- [sort_unstable](sort-unstable.md)
 - [specialization](specialization.md)
 - [staged_api](staged-api.md)
 - [start](start.md)

src/doc/unstable-book/src/sort-unstable.md

@@ -0,0 +1,9 @@
+# `sort_unstable`
+
+The tracking issue for this feature is: [#40585]
+
+[#40585]: https://github.com/rust-lang/rust/issues/40585
+
+------------------------
+
+