Add learning csharp ebook to books (#10196)

* Add learning csharp ebook to books

* Changes after PR comment

---------

Co-authored-by: Tom <t.baten@sigmax.nl>

books/free-programming-books-langs.md

@@ -494,6 +494,7 @@ Books on general-purpose programming that don't focus on a specific language are
 * [High level asynchronous programming with Reactive Extensions](https://github.com/petroemil/Rx.Book) - Emil Petro
 * [Introduction to Rx](http://www.introtorx.com)
 * [Learn C# in Y Minutes](https://learnxinyminutes.com/docs/csharp/)
+* [Learning C# Language](https://riptutorial.com/Download/csharp-language.pdf) - Compiled from StackOverflow Documentation (PDF)
 * [Mastering Xamarin UI Development, Second Edition](https://www.packtpub.com/free-ebook/mastering-xamarin-ui-development-second-edition/9781788995511) - Steven F. Daniel (Packt account *required*)
 * [Modernize existing .NET applications with Azure cloud and Windows Containers](https://docs.microsoft.com/en-us/dotnet/architecture/modernize-with-azure-containers/) - Cesar de la Torre
 * [Modernizing Desktop Apps on Windows with .NET 6](https://docs.microsoft.com/en-us/dotnet/architecture/modernize-desktop) - Olia Gavrysh, Miguel Angel Castejón Dominguez