docs: document spec.clusterDomain in configuration guide

[arekborucki]

Why

spec.clusterDomain existed only in the API reference, with no explanation of when or why to set it. Users on clusters with a non-default kubelet --cluster-domain had no guidance, and a mismatch can break. Keeper coordination and Distributed queries with DNS resolution errors.

What

Adds a Cluster domain section to the Configuration guide covering:

• the default value, cluster.local
• the FQDN format generated by the operator
• where the value is used (remote_servers replica host names and Keeper coordination host names)
• the requirement to set the same value on both ClickHouseCluster and KeeperCluster

[mintlify]

Docs PR opened: ClickHouse/mintlify-docs-dev#209

Added a Cluster domain section to the Kubernetes Operator configuration guide explaining when and how to override spec.clusterDomain.