Add 'RTL Styling 101' guide (#6461)

* Add 'RTL Styling 101' guide

* Fix 'RTL Styling 101' URL linting issue
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -937,6 +937,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Pocket Guide to Writing SVG](https://svgpocketguide.com) - Joni Trythall
 * [Pro HTML5 Programming](https://web.archive.org/web/20181215200026/http://apress.jensimmons.com/v5/pro-html5-programming/ch0.html) - Jen Simmons, Chris O'Connor, Dylan Wooters, Peter Lubbers
 * [Resilient Web Design](https://resilientwebdesign.com/#Resilientweb%20design) - Jeremy Keith
+* [RTL Styling 101](https://rtlstyling.com) - Ahmad Shadeed
 * [Scalable and Modular Architecture for CSS](https://web.archive.org/web/20191116073929/http://smacss.com/) - Jonathan Snook
 * [Understanding Flexbox: Everything you need to know](https://ohansemmanuel.github.io/uf_download.html) - Ohans Emmanuel
 * [W3.CSS Succinctly](https://www.syncfusion.com/ebooks/w3_css_succinctly) - Joseph D. Booth