The catalog's architecture is scattered across readme files in different folders:

• The sharding architecture guide under mongo/db/s, which covers the distributed aspects of the catalog and routing. However it also includes content unrelated to the catalog, like transactions management and data movement.
• The execution architecture guide under mongo/db/catalog, which covers the process-local part of the catalog, among a majority of unrelated storage execution content.

This ticket is to consolidate this content into a dedicated catalog architecture guide. This is also an opportunity to provide a high-level view of the MongoDB catalog: how it's layered and deployed in a MongoDB cluster, the isolation levels it offers, and the consistency model it uses.