Rewrite concept section of auto-merge page

What does this MR do?

Reopen when work is happening in Automatically merge when ready (merge checks an... (#8128 - closed)

Begins to rewrite the concept (top) section of the auto-merge (formerly MWPS) page: https://docs.gitlab.com/ee/user/project/merge_requests/merge_when_pipeline_succeeds.html

I reached some concepts I wasn't sure on, and realized it was time to stop and get a fact check.

Related issues

• Related to Plan for changes from 'auto_merge_labels_mr_wid... (technical-writing#782) - this page is a known big piece of the work for Create
• Related to Replace merge strategy labels with `Auto-merge`... (#359057 - closed)
• Related to Automatically merge when ready (merge checks an... (#8128 - closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Documentation guidelines.
  • Style Guide.
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If you are a GitLab team member and 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.

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

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