Add architecture blueprints for Cloud Connector

What does this MR do and why?

Add architecture blueprints for GitLab Cloud Connector.

Note that this focuses on the status quo, specifically Code Suggestions for self-managed users, which was the MVC for Cloud Connector (formerly: GitLab Plus). As such, this is not so much a proposal but an attempt at documenting what has already been built and shipped, and what will serve as a basis for future iterations. While I appreciate any feedback on potential issues with this approach, I would prefer to address that separately from just documenting facts.

The intention here is:

• Provide an overview page that describes Cloud Connector and lists individual blueprints related to its enclosed use cases. This is necessarily incomplete currently as we are still working out what Cloud Connector will look like from a product perspective.
• Provide an account of the Cloud Connector MVC architecture, which was delivering Code Suggestions to self-managed users.

Deep links to docs preview

• https://main-ee-129610.docs.gitlab-review.app/ee/architecture/blueprints/cloud_connector/index.html
• https://main-ee-129610.docs.gitlab-review.app/ee/architecture/blueprints/cloud_connector/cloud_connector_mvc.html

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.

Related to #422369 (closed)