Rename MWPS file to auto-merge in global nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!145872 (merged) moves and shifts the content of doc/user/project/merge_requests/merge_when_pipeline_succeeds.md to auto_merge.md
• Give the MR a descriptive title.
• Review app: https://main-ee-145872.docs.gitlab-review.app/ee/user/project/merge_requests/auto_merge.html

Pipeline will not pass until gitlab!145872 (merged) merges.

Related issues

• Related to Docs follow-up: Merge When Checks Pass (gitlab#440696 - closed) and may even close it
• Related to Document new Merge When Checks Pass (gitlab!143479 - merged) where these changes were requested but not done
• Related to #431449+
• Related to Plan for changes from 'auto_merge_labels_mr_wid... (technical-writing#782)
• Related to [Feature flag] Rollout of `merge_when_checks_pass` (gitlab#412995)
• Related to [Feature flag] Rollout of `additional_merge_whe... (gitlab#424784 - closed)