Docs: explain how review apps work for the docs site

What

Document how the docs site review apps work.

Why

We use Docker images to generate the review app for CE and EE, and we need this process documented to be able to improve the structure and troubleshoot in case something goes wrong.

Recently we broke all the pipelines in CE and EE due to a late Docker image's update (https://gitlab.com/gitlab-org/gitlab-ce/issues/55038).

Where

Under development/documentation/site_architecture/, available when https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23065 gets merged.