Docs: Fixing broken cyclomatic complexity link (fixes #8396) (#8937)

Also added a few more useful links.

docs/rules/complexity.md

@@ -70,8 +70,11 @@ If you can't determine an appropriate complexity limit for your code, then it's
 
 ## Further Reading
 
-* [About Complexity](http://jscomplexity.org/complexity)
+* [Cyclomatic Complexity](https://en.wikipedia.org/wiki/Cyclomatic_complexity)
 * [Complexity Analysis of JavaScript Code](http://ariya.ofilabs.com/2012/12/complexity-analysis-of-javascript-code.html)
+* [More about Complexity in JavaScript](https://craftsmanshipforsoftware.com/2015/05/25/complexity-for-javascript/)
+* [About Complexity](https://web.archive.org/web/20160808115119/http://jscomplexity.org/complexity)
+* [Discussion about Complexity in ESLint and more links](https://github.com/eslint/eslint/issues/4808#issuecomment-167795140)
 
 ## Related Rules