Update Gambas links, fixes #2523 (#2586)

* Update Gambas links

Updated broken Gambas beginner's guide link, and add additional link to Gambas resources since the original book's website has been taken down

* removed gambas links

Not sure the book is free anymore, and realised second link doesn't comply with contribution guidelines as it's not an actual book

* Re-add beginner's guide to gambas

free-programming-books.md

@@ -846,7 +846,7 @@
 ### Basic
 
 * [10 PRINT CHR$(205.5+RND(1)); : GOTO 10](http://10print.org) - Nick Montfort, Patsy Baudoin, John Bell,Ian Bogost, Jeremy Douglass, Mark C. Marino, Michael Mateas, Casey Reas, Mark Sample, Noah Vawter
-* [A beginner's guide to Gambas](http://beginnersguidetogambas.com)
+* [A beginner's guide to Gambas](http://distro.ibiblio.org/vectorlinux/Uelsk8s/GAMBAS/gambas-beginner-guide.pdf) - John W. Rittinghouse (PDF)
 * [Pick/Basic: A Programmer's Guide](http://www.jes.com/pb/) - Jonathan E. Sisk
 * [Visual Basic Essentials](http://www.techotopia.com/index.php/Visual_Basic_Essentials)