Add Math Books from aimath.org (#5256)

* Add Math Books from aimath.org

* Fix Trailing Slashes

* Fix Alphabetical Order?

* Fix Alphabetical Order

* Fixed language and categories

* Fixed language and categories for -ko.md

* Removed most PDF links from addition.

* Replace German Resource with Direct Link to PDF

books/free-programming-books-es.md

@@ -28,6 +28,7 @@
   * [React](#react)
 * [LaTeX](#latex)
 * [Linux](#linux)
+* [Matemáticas](#matem%C3%A1ticas)
 * [.NET (C# Visual Studio)](#net-c--visual-studio)
 * [NoSQL](#nosql)
   * [MongoDB](#mongodb)
@@ -256,6 +257,11 @@
 * [El Manual del Administrador de Debian](https://debian-handbook.info/browse/es-ES/stable/) (HTML) [(PDF, ePub, Mobi)](https://debian-handbook.info/get/now/)
 
 
+### Matemáticas
+
+* [Sage para Estudiantes de Pregrado](http://www.sage-para-estudiantes.com) - Gregory Bard
+
+
 ### .NET (C# / Visual Studio)
 
 * [El lenguaje de programación C#](http://dis.um.es/~bmoros/privado/bibliografia/LibroCsharp.pdf) (PDF)