docs: improve merge request template

Dmitry Gruzdrequested to merge
docs/improve-merge-request-template into main

What does this MR do and why?

Updates the default merge request template to keep reviewer-facing sections concise and move implementation details into a collapsible Agent context block. This follows the structure used by the Orbit Knowledge Graph template.

Testing

Template-only change. Verified the Markdown structure, quick actions, project labels, and whitespace locally.

Performance analysis

  • This merge request does not introduce a performance regression.

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