docs: general fixes across documentation (#4248)

* docs: added details about the migration revert command

* added a note about running postgres before the backend

* added a link to typeorm's documentation

* fix broken links

* fixed section title

* fixed admonitions

docs/content/development/entities/create.md

@@ -177,6 +177,12 @@ await postRepository.softDelete(post.id)
 
 ---
 
+## Advanced Entity Definitions
+
+With entities, you can create relationships, index keys, and more. As Medusa uses Typeorm, you can learn about using these functionalities through [Typeorm's documentation](https://typeorm.io/).
+
+---
+
 ## See Also
 
 - [Extend Entity](./extend-entity.md)