[docs] Add references to Kubernetes in SkyServe docs

[romilbhardwaj]

          nit: should we also mention serving on kubernetes a bit in the serving doc and refer to the cloud-permissions for the setup?

We may also want to explicitly list out why people should use skyserve on kubernetes than just using deployments, such as:

1. easier customization for load balancer and autoscaling policy
2. simple interface and easy to setup
3. spillover to clouds.

Originally posted by @Michaelvll in #3377 (comment)

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