Rewrite Cloud Connector architecture docs

What does this MR do and why?

Rewrite Cloud Connector architecture docs.

This replaces the original docs that were written with the Code Suggestions MVC and AI in mind. The target audience is GitLab team members and contributors, not customers/users.

Preview: https://main-ee-143538.docs.gitlab-review.app/ee/development/cloud_connector/architecture.html

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Related to #439854 (closed)