feat: Add Atcoder and CSES Competitive Programming Resources (#12335)

* feat: Add Atcoder and CSES problem sets

* fix: Correct alphabetical order

* fix: Move resources to correct file and add new resource

* fix: Add final newline character

more/problem-sets-competitive-programming.md

@@ -15,6 +15,7 @@
 * [Algorithms for Competitive Programming](https://cp-algorithms.com)
 * [APL Problem Solving Competition](https://contest.dyalog.com)
 * [AtCoder](https://atcoder.jp)
+* [Atcoder DP Contest](https://atcoder.jp/contests/dp) - Atcoder
 * [beecrowd](https://www.beecrowd.com.br)
 * [COCI](https://hsin.hr/coci/)
 * [Codeabbey](http://www.codeabbey.com)
@@ -155,4 +156,3 @@
 * [UVa Online Judge](https://uva.onlinejudge.org/index.php?Itemid=8&option=com_onlinejudge)
 * [Vulnhub.com](https://www.vulnhub.com)
 * [Школа программиста](https://acmp.ru)
-