GitLab CI Events ADR 001: Use hierarchical events

What does this MR do and why?

This MR adds the first Architectural Decision Record into GitLab CI Events design doc: decision to use hierarchical events.

See Decouple design doc vision from decisions record (#424486 - closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.