*This PR:
Copies the existing clusters API reference to the new structure. The reference docs are split out into Ray Clusters (common between vms and k8s) and Ray Clusters on VMs (specific to vms). Notably, there is also a reference section for k8s, but not in this PR.
Move the three job submission user guides back into a single one. Jules had suggested that we break them out into rest/sdk/cli, but that's not P0 right now.
Fix some bugs in the left navigation bar. There should be less duplication of TOC entries. I'll keep working on related fixes in a different PR.
Signed-off-by: Cade Daniel <cade@anyscale.com>
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