Ensure routing_rules setting is the same across all nodes

What does this MR do?

Explicitly state that routing rules setting should be the same across all Rails and Sidekiq nodes, in the elasticsearch tuning doc. If Rails aren't using the same routing rules, Rails still route jobs to individual queues based on worker name (assuming empty routing rules), i.e. elastic_commit_indexer, authorized_projects, etc (there are 600 of these queues), but some Sidekiq nodes only listen to default,mailers queues and the other nodes only listen to global_search (which is a made-up hardcoded name).

This came up from a customer issue where jobs are piling up in the queue after changing routing rules and running the Rake task to migrate jobs.

Related issues

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