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-de.md

@@ -17,6 +17,7 @@
 * [JavaScript](#javascript)
   * [React](#react)
 * [LaTeX](#latex)
+* [Mathematik](#mathematik)
 * [Meta-Lists](#meta-lists)
 * [MySQL](#mysql)
 * [Neo4j](#neo4j)
@@ -149,6 +150,11 @@
 * [LaTeX : Referenz der Umgebungen, Makros, Längen und Zähler](http://www.lehmanns.de/page/latexreferenz)
 
 
+### Mathematik
+
+* [Calcul mathématique avec SAGE](http://www.loria.fr/~zimmerma/sagebook/CalculDeutsch.pdf) - Paul Zimmermann et al. (PDF)
+
+
 ### Meta-Lists
 
 * [Galileo Computing - openbook](https://www.rheinwerk-verlag.de/openbook)