fix: Use Markdown instead of HTML markup for H3 & H4 headings (#6689)

* fix: Markdown markup for `C++` heading

* fix: Markdown markup for `Дизайн и Aрхитектура` heading

* fix: Markdown H4 markup for `Next.js` heading

* fix: Markdown H3 markup for `C#` heading

books/free-programming-books-langs.md

@@ -28,7 +28,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [BeanShell](#beanshell)
 * [BETA](#beta)
 * [C](#c)
-* [C#](#c-sharp)
+* [C#](#csharp)
 * [C++](#cpp)
 * [Chapel](#chapel)
 * [Cilk](#cilk)
@@ -462,7 +462,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [TONC GBA Programming - Game Boy Advance Development](http://www.coranac.com/tonc/text/toc.htm)
 
 
-### C Sharp
+### <a name="csharp"></a>C\#
 
 * [C# Notes for Professionals](http://goalkicker.com/CSharpBook/) - Compiled from StackOverflow documentation (PDF)
 * [C# Programming](https://en.wikibooks.org/wiki/C_Sharp_Programming) - Wikibooks
@@ -485,7 +485,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Xamarin.Forms Succinctly](https://www.syncfusion.com/ebooks/xamarin-forms-succinctly) - Alessandro Del Sole
 
 
-<h3 id="cpp">C++</h3>
+### <a name="cpp"></a>C++
 
 * [C++ Annotations](https://fbb-git.gitlab.io/cppannotations/) - Frank B. Brokken (HTML, PDF)
 * [C++ Core Guidelines](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md) - Editors: Bjarne Stroustrup, Herb Sutter