docs: section 11 of the storage design still lists runtime backend selection #891

Section 11.8 of docs/design/04-component-storage.md lists "Runtime backend selection (choose backend from config file)" as a reason for trait objects. Each binary has one backend, and the build chooses it. crates/bin/src/main.rs stops the build with compile_error! unless exactly one backend feature is on, and main installs that backend with set_backend (crates/storage/src/backend.rs).

Section 11 also has no step for a backend that cannot pin a table snapshot. Since #700, such a backend needs an entry in in_catalog_create_only in crates/server/src/backup_store.rs, so that its CreateBackup writes the in-catalog format. #668 adds this to section 1 of the same file.