Added some explanation to relational and MongoDB schemas

Author: dfreniche

docs/30-schema-in-postgresql.mdx

@@ -2,6 +2,8 @@ import Screenshot from "@site/src/components/Screenshot";
 
 # 📘 Schema in PostgreSQL
 
-The following screenshot shows an Entity-Relationship diagram of the relational database we wish to migrate.
+The following screenshot shows an Entity-Relationship diagram of the relational database we wish to migrate. This is a Library management app, that stores information about Books, where a Book can have many Authors, an Author can write many books, users can borrow books and leave reviews.
+
+This is the sample schema we use during our [Developer Days's Intro Lab](https://mongodb-developer.github.io/intro-lab/docs/importing-data/intro).
 
 <Screenshot url="https://www.mongodb.com/products/tools/relational-migrator" src="img/30-image-001.png" alt="Screenshot of the connect modal" />

docs/40-desired-schema-mongodb.mdx

@@ -9,3 +9,8 @@ that data that is frequently accessed together is stored together by using embed
 The following screenshot shows the MongoDB schema we will be creating in this lab.
 
 <Screenshot url="https://www.mongodb.com/products/tools/relational-migrator" src="img/40-image-001.png" alt="Screenshot of the connect modal" />
+
+A few key points:
+- books will contain an array of authors with author id and name for fast retrieval. This is using the [Extended Reference Pattern](https://www.mongodb.com/blog/post/building-with-patterns-the-extended-reference-pattern) to reference a separate Collection (Authors) but having in books enough information aubout the author.
+- reviews are not included in the books collection as they can grow and become an [unbounded array](https://www.mongodb.com/developer/products/mongodb/schema-design-anti-pattern-massive-arrays/), an atipattern in Schema Design.
+- an author can have several aliases, we just store those strings in an array.