Add Cloud Connector ADR 001 to design doc

What does this MR do and why?

Add Cloud Connector ADR 001, which documents a temporary reduction in scope by deciding to stand up an additional load balancing/routing layer at cloud.gitlab.com instead of a dedicated edge service, while leaving the doors open to build such a service at a later point in time.

Also removed the confusing downstream/upstream language from the original BP by just saying "backend".

Preview:

https://main-ee-135818.docs.gitlab-review.app/ee/architecture/blueprints/cloud_connector/decisions/001_lb_entry_point.html

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