Added LaTex Notes for Professionals (#6418)

I added LaTex Notes for Professionals from goalkicker compiled from StackOverflow documentation in .pdf format.

books/free-programming-books-langs.md

@@ -1367,6 +1367,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Arbitrary LaTex Reference](http://latex.knobs-dials.com)
 * [Begin Latex in minutes](https://github.com/VoLuong/Begin-Latex-in-minutes)
 * [LaTeX](https://en.wikibooks.org/wiki/LaTeX) - Wikibooks
+* [LaTex Notes for Professionals](https://goalkicker.com/LaTeXBook/) - Compiled from StackOverflow documentation (PDF)
 * [The Not So Short Introduction to LaTeX](https://tobi.oetiker.ch/lshort/lshort.pdf) (PDF)