This PR:
Creates a new chapter in the docs titled "Ray Clusters (Under Construction)".
The new chapter makes the Ray Clusters docs follow the same structure as the other docs (https://diataxis.fr/)
The new chapter will eventually replace the old chapter.
I want to merge this now so that @DmitriGekhtman can put his Kuberay docs into the new structure.
Signed-off-by: Cade Daniel <cade@anyscale.com>
