Add Jenkins User Handbook (official) (#5749)

books/free-programming-books-langs.md

@@ -1267,6 +1267,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 ### Jenkins
 
 * [Jenkins: The Definitive Guide](http://www.bogotobogo.com/DevOps/Jenkins/images/Intro_install/jenkins-the-definitive-guide.pdf) (PDF)
+* [Jenkins User Handbook](https://www.jenkins.io/user-handbook.pdf) (PDF)
 
 
 ### Julia