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-ko.md

@@ -1,5 +1,6 @@
 ### Index
 
+* [수학](#%EC%88%98%ED%95%99)
 * [Amazon Web Service](#amazon-web-service)
 * [Assembly Language](#assembly-language)
 * [C](#c)
@@ -27,6 +28,11 @@
 * [Swift](#swift)
 
 
+### 수학
+
+* [기초정수론: 계산과 법연산, 그리고 비밀통신을 강조한](https://wstein.org/ent/) - William Stein
+
+
 ### Amazon Web Service
 
 * [아마존 웹 서비스를 다루는 기술](http://www.pyrasis.com/private/2014/09/30/publish-the-art-of-amazon-web-services-book)