Rename to "auto-merge" first, easy round of fixes

What does this MR do?

Now that Add note that MWPS is now auto-merge (!120525 - merged) is merged, and we're waiting on the feature flag for Update MWPS/MT/MTWPS to auto-merge in across docs (#394752 - closed), let's prepare the merge request with a first, easy round of fixes for the MWPS string. Images and more complex usage are not handled here, though I'm being brave and tackling some subheading renames...

Related issues

• Related to Update MWPS/MT/MTWPS to auto-merge in across docs (#394752 - closed)
• Related to Replace merge strategy labels with `Auto-merge`... (#359057 - closed)
• Depends on Enables the auto_merge_labels_mr_widget feature... (!120649 - merged) enabling the changes for users.

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