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-nl.md

@@ -1,7 +1,7 @@
 ### Index
 
 * [C](#c)
-* [C#](#c-sharp)
+* [C#](#csharp)
 * [COBOL](#cobol)
 * [Java](#java)
 * [PHP](#php)
@@ -15,7 +15,7 @@
 * [Programmeren in C](https://nl.wikibooks.org/wiki/Programmeren_in_C) - Wikibooks
 
 
-### C Sharp
+### <a name="csharp"></a>C\#
 
 * [Programmeren in C Sharp](https://nl.wikibooks.org/wiki/Programmeren_in_C_Sharp) - Wikibooks