[DOCS] uni-directional CCR disaster recovery

[Leaf-Lin]

Detailed step-by-step tutorial for uni-directional CCR disaster recovery

closes: https://github.com/elastic/elasticsearch/issues/84854

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[henningandersen]

@elasticmachine run elasticsearch-ci/docs @elasticmachine run elasticsearch-ci/docs-check

[Leaf-Lin]

@elasticmachine run elasticsearch-ci/docs @elasticmachine run elasticsearch-ci/docs-check

[lockewritesdocs]

I added this file to the table of contents so that it displays in the navigation. I also made changes to the introductory section, but I have questions about how this tutorial works. We make a lot of assumptions in this document. For example, the first step says:

1. On the Clients side, pause ingestion of my_index into prod1.

TIP: Ensure no writes are occurring on the leader index (if the data centre is down, or cluster is unavailable, no action needed).

How do you pause ingestion? Through Kibana? If so, where? What is "the Clients side"? How can I tell if writes are occurring on the leader index or check the cluster's availability?

I don't think that we can assume users have all of this knowledge and instinctually know where to complete these tasks. We should be much more prescriptive and provide both direction (such as where to go in the Cloud console) and examples.

@Leaf-Lin, can you help to provide some additional context?

[Leaf-Lin]

I've made some changes to this doc including clarifying how to pause ingest on the client side and adding an image to clarify the 3 scenarios.

Preview page is available in: https://elasticsearch_87099.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/ccr-disaster-recovery-uni-directional-tutorial.html

Btw, I only changed this one page, but the https://elasticsearch_87099.docs-preview.app.elstc.co/diff shows a lot of changes, I hope that won't be merged.

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[Leaf-Lin]

Closing this in favour of https://github.com/elastic/elasticsearch/pull/91491