ECH - Promote KB about allocator moving nodes due to essential system maintenance

We have to promote that KB to our public docs.
Public link of the KB: https://support.elastic.co/knowledge/028f4337

The original plan is to integrate the content into https://www.elastic.co/docs/troubleshoot/monitoring/node-moves-outages

Details of the proposed plan and motivations are available in the private issue linked to this one.

Thanks @kunisen for sharing this!