Added multi-nodes/processes Sidekiq sections

What does this MR do?

It adds a section where we explain how customers can add extra Sidekiq processes or nodes to isolate the indexing queues from the rest of the Sidekiq queues.

This is necessary because sometimes indexing takes over Sidekiq and indexing (a process that is not time-critical or vital) can overwhelm Gitlab and actually crash it...and in so doing affecting end-users

Related issues

#351600 (closed)

#339744 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
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  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
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• Ensure a release milestone is set.