docs: add procedure for reconfiguring etcd mode (server ↔ client)

[tsivaprasad]

Summary

This PR adds detailed documentation and verified procedures for changing a Control Plane host’s etcd mode between server and client after initialization.

Changes

• Steps to safely promote a client → server
• Steps to safely demote a server → client
• Required environment variable changes
• Performed full manual verification in a dev cluster

Testing

Manually validated:

• Promoting a client host to server mode
• Demoting a server host to client mode

Verified etcd cluster state with:

• etcdctl member list -w table
• etcdctl endpoint health

Checklist

• Documentation updated

PLAT-315

[Copilot]

Pull request overview

This PR adds comprehensive documentation for reconfiguring etcd mode on Control Plane hosts, enabling operators to safely change hosts between server and client modes after initial deployment. The documentation includes detailed procedures, troubleshooting guidance, and best practices based on manual testing in a development environment.

Key Changes

• Added complete step-by-step procedures for promoting clients to servers and demoting servers to clients
• Documented environment variable configuration requirements and container restart workflows
• Fixed a bug in rbac.go to ensure EtcdMode is persisted when writing host credentials

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
docs/using/etcd-reconfiguration.md	New documentation file covering etcd mode reconfiguration procedures, prerequisites, troubleshooting, and best practices
server/internal/etcd/rbac.go	Added missing EtcdMode assignment to ensure mode is preserved in generated config

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