Document remote cluster setup with non-eck clusters

[strawgate]

With #8167 we added the ability to easily configure CCS between two eck clusters

What's less clear is if and how a user who wants to use an eck cluster as a remote cluster would configure it.

The documentation we provide leads the user to setting up certificate based CCS "Connect from an Elasticsearch cluster running outside the Kubernetes cluster" but it doesn't look like we tell the user they need to setup client certificates https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-remote-clusters.html

We should document the setup for connecting a cluster from outside of eck using API keys and rename the current section to be for certificate based ccs