Refactor the Database Object Permissions guide

Closes #41917

Merge the Database Object Permissions guide into the Database Access
RBAC guide for greater discoverability and a clearer division of labor
between the two guides.

This change also includes the following edits to make the refactor
cleaner, since we can include each troubleshooting step as a separate H3
in the dateabase object permissions H2:

- Remove an unnecessary troubleshooting step: One step indicates that
  import rules are validated, which is unnecessary to document, since
  validation errors are self explanatory.
- Instead of mentioning the admin user as a troubleshooting step, add a
  separate H3 for the admin user and describe the `admin_user` field,
  which was not mentioned in the original database object permissions
  guide.

docs/config.json

@@ -1268,10 +1268,6 @@
             {
               "title": "PostgreSQL",
               "slug": "/database-access/auto-user-provisioning/postgres/"
-            },
-            {
-              "title": "Database Access Controls",
-              "slug": "/database-access/auto-user-provisioning/database-access-controls/"
             }
           ]
         },