Added Java Sinhala tutorial (#9340)

* Added Java Sinhala tutorial

* Added Java Sinhala tutorial

* Added REST API with Spring Boot Sinhala tutorial

* Updates

---------

Co-authored-by: Mahesh-Abeykoon <mahesh.a@affiniti.biz>

courses/free-courses-si.md

@@ -4,6 +4,7 @@
 * [C#](#csharp)
 * [HTML and CSS](#html-and-css)
 * [Java](#java)
+    * [Spring Boot](#spring-boot)
 * [JavaScript](#javascript)
     * [React](#react)
 
@@ -25,6 +26,7 @@
 
 ### Java
 
+* [Introduction to Java](https://www.youtube.com/playlist?list=PLuhSdp06EMkLgaWqSPZKLqePVw-dtqaTT) - Masith Prasanga
 * [Object Oriented Programming ](https://youtube.com/playlist?list=PLqeCu_1ZdDl63h6YR3QsxcGOB7yDS7i3b) - LankaDroid Programming Kuppiya (YouTube)
 * [Sinhala Java Netbeans Lessons](https://youtube.com/playlist?list=PLA3ZeQncjeVu9VHevp2SmPCQ9muVO3fEB) - Chanux Bro
 
@@ -40,3 +42,7 @@
 * [MERN Stack Developer - Beginners](https://www.youtube.com/playlist?list=PLvfC6i-hEZBnqqF7giszuYI0iqenU5NY0) -  TechWithGeorge
 * [REACT | MERN CRUD App in Sinhala](https://youtube.com/playlist?list=PLtoqJbwHBeHzAooLCGOzYVE9mkAeCnT9y) - Haritha Weerathunga
 
+
+#### Spring Boot
+
+* [REST API with Spring Boot](https://www.youtube.com/playlist?list=PLuhSdp06EMkIhKEo_H-IjrG0cozCuS9lE) - Masith Prasanga