docs: Add instructions for Ceph shutdown

[Alex-Welsh]

Client has a full shutdown coming up, which they are handling themselves. Most of the guide is fine but the Ceph section was quite sparse.

Renderd outupt:

[gemini-code-assist]

Summary of Changes

Hello @Alex-Welsh, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the control-plane-operation.rst documentation by providing detailed instructions for safely shutting down a Ceph cluster. The update addresses a previous sparsity in the guide, offering a clear, step-by-step process to assist clients with planned full system shutdowns. The changes aim to ensure data integrity and smooth operation during maintenance periods by outlining critical pre-shutdown checks and proper shutdown sequences.

Highlights

• Ceph Shutdown Documentation: Added a comprehensive, step-by-step guide for safely shutting down a Ceph cluster, including checks for cluster health, stopping clients, setting the noout flag, and the order of node shutdown.
• Updated External Reference: Replaced an outdated Red Hat documentation link with a more relevant blog post from croit.io on Ceph shutdown best practices.

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[gemini-code-assist]

Code Review

This pull request adds much-needed documentation for shutting down a Ceph cluster. The instructions are a good start. I've provided a few suggestions to improve clarity and correctness. Specifically, I've pointed out a small typo, recommended adding the explicit command for setting the noout flag for better usability, and highlighted a critical omission regarding the need to unset the noout flag during startup to ensure the cluster returns to a healthy state. Addressing these points will make the documentation more complete and safer for operators.

Changes applied