Document new Merge When Checks Pass

What does this MR do and why?

Before turning on the feature flag to enable Merge when checks pass for the whole SaaS, we should add documentation around it.

Related issues

• Related to Document new Merge When Checks Pass (#431449 - closed)
• Related to Plan for changes from 'auto_merge_labels_mr_wid... (technical-writing#782)
• Related to [Feature flag] Rollout of `merge_when_checks_pass` (#412995)
• Related to [Feature flag] Rollout of `additional_merge_whe... (#424784 - closed)