Add Vogel's Using RxJava 2 Tutorial (#5439)

* Add Vogel's Using RxJava 2 Tutorial

Source: https://www.vogella.com/tutorials/RxJava/article.html

* Move Vogel's Using RxJava 2 Tutorial to books

- Move from tutorials to books as suggested in PR.

* Move Vogel's Using RxJava 2 Tutorial to books

- Revert VSCode auto-markdown

books/free-programming-books.md

@@ -1712,6 +1712,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Think Data Structures: Algorithms and Information Retrieval in Java](https://greenteapress.com/wp/think-data-structures/) - Allen B. Downey (PDF, HTML)
 * [Think Java: How to Think Like a Computer Scientist, 2nd Edition](https://greenteapress.com/wp/think-java-2e/) - Allen B. Downey and Chris Mayfield (HTML, PDF) [(Interactive version by Trinket)](https://books.trinket.io/thinkjava2/)
   * [Think Java: How to Think Like a Computer Scientist](https://greenteapress.com/wp/think-java/) - Allen B. Downey and Chris Mayfield (HTML, PDF) [(Interactive version by Trinket)](https://books.trinket.io/thinkjava/)
+* [Using RxJava 2 Tutorial](https://www.vogella.com/tutorials/RxJava/article.html) - Lars Vogel, Simon Scholz (HTML)
 * [Welcome to Java for Python Programmers](https://runestone.academy/runestone/books/published/java4python/index.html) - Brad Miller
 * [Welcome to the Java Workshop (2006)](http://javaworkshop.sourceforge.net) - Trevor Miller
 * [What’s New in Java 8](https://leanpub.com/whatsnewinjava8/read) - Adam L. Davis