docs: add a note about running migrations after changing database type (#2699)

* Add note to run migrations

I think it will be helpful to remind newbies to run migrations after they change their `database_type`.  I'm a newbie and today I got stuck for a while because I didn't know I had to run migrations after I changed from `sqlite` to `postgres`.

* Add H3 header "Changing Database Type"

docs/content/usage/configurations.md

@@ -80,6 +80,14 @@ DATABASE_URL=<YOUR_DATABASE_URL>
 
 Where `<YOUR_DATABASE_URL>` is the URL of your PostgreSQL database.
 
+### Changing Database Type
+
+Remember to run migrations after you change your database type to `postgres` from another type:
+
+```bash
+medusa migrations run
+```
+
 ### Common Configuration
 
 As Medusa internally uses [Typeorm](https://typeorm.io/) to connect to the database, the following configurations are also available: