HDDS-15031. Add troubleshooting page for slow datanode decommission

[Eason09053360]

What changes were proposed in this pull request?

This fills in the Troubleshooting → Decommissioning and Maintenance page, which was a draft: true placeholder under HDDS-9860.

The page covers a Datanode decommission that is slow or stuck:

• How the SCM admin monitor decides a node is done (pipelines closed, state confirmed by heartbeat, every container sufficiently replicated) and how the Replication Manager throttles the copies.
• How to check progress with ozone admin datanode status decommission, the per-node SCM log summary line, and the NodeDecommissionMetrics / ReplicationManagerMetrics JMX metrics.
• Common causes for a stuck workflow: pipelines that do not close, containers that do not close, no suitable target, no source with capacity, replication command timeouts, and the node dying mid-workflow, each with the SCM log message to look for.
• The four replication throttles (hdds.scm.replication.datanode.replication.limit, hdds.datanode.replication.outofservice.limit.factor, hdds.scm.replication.inflight.limit.factor, hdds.datanode.replication.streams.limit), their defaults, where they apply, and which are reconfigurable at runtime.

Property defaults, log messages, metric names and CLI output were checked against the current Ozone source.

The Datanode Decommission page in the administrator guide gets a "See also" link to the new page, since nothing linked to it before.

What is the link to the Apache Jira?

https://issues.apache.org/jira/browse/HDDS-15031

How was this patch tested?

• markdownlint on the two changed pages.
• npm run build, which fails on broken links and anchors in this site, completes successfully.
• Checked the rendered page locally.

Generated-by: Claude Code (Claude Fable 5.1), used to cross-check the page against the Ozone source and to apply the resulting fixes.