docs: add section on idempotent CREATE TYPE

Created by: Strum355

Should be explained in the docs. Example failure can be seen here https://buildkite.com/sourcegraph/sourcegraph/builds/149723#0180fbf8-e506-4b99-9b29-febe1515fb37 with a search for cannot drop type audit_log_operation because other objects depend on it (SQLSTATE 2BP01)

Test plan

N/A docs change