[docs] Placement group documentation (#10555)

Co-authored-by: Richard Liaw <rliaw@berkeley.edu>

doc/source/using-ray.rst

@@ -16,8 +16,8 @@ Finally, we've also included some content on using core Ray APIs with `Tensorflo
    using-ray-with-gpus.rst
    serialization.rst
    memory-management.rst
+   placement-group.rst
    troubleshooting.rst
-   ray-dashboard.rst
    advanced.rst
    cross-language.rst
    using-ray-with-tensorflow.rst