Post-merge review for changes to webhook page

What does this MR do?

Merge request !197850 (merged) had a difficult review process (91 comments!) and I offered to handle the docs changes in a follow-up merge request. The request for a post-merge review was moved to #570057 (closed).

The changes tidy up the edits to doc/user/project/integrations/webhook_events.md, and include:

• Alphabetizing a table, starting around line 28
• Straightening up table columns
• Some polish to the new ### System-initiated merge request events subheading around line 917

Related issues

• Closes Polish recent changes to webhook-triggers page (#570057 - closed)
• Related to Add webhook trigger for system-initiated approv... (!197850 - merged)
• Related to Merge request webhook when approvals are reset ... (#553070 - closed)

Author's checklist

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Reviewer's checklist

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