chore: add resource under postgresql-dba:introduction:what-are-relational-databases

src/data/roadmaps/postgresql-dba/content/101-introduction/100-what-are-relational-databases.md

@@ -27,4 +27,6 @@ Relational databases offer several advantages in terms of efficiency, flexibilit
 - **Scalability**: Relational databases can handle large amounts of structured data and can be scaled to accommodate growing data requirements.
 - **Querying**: The SQL (Structured Query Language) is used for querying, updating, and managing relational databases, providing a powerful and standardized way to access and manipulate the data.
 
-In summary, relational databases are a powerful and versatile tool for storing and managing structured data. Their ability to represent relationships among data and to ensure data integrity make them the backbone of many applications and services.
+In summary, relational databases are a powerful and versatile tool for storing and managing structured data. Their ability to represent relationships among data and to ensure data integrity make them the backbone of many applications and services.
+
+- [Relational Databases: concept and history](https://www.ibm.com/topics/relational-databases)