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-dk.md

@@ -1,7 +1,7 @@
 ### Index
 
 * [C](#c)
-* [C#](#c-sharp)
+* [C#](#csharp)
 * [C++](#cpp)
 * [Java](#java)
 * [Pascal](#pascal)
@@ -12,12 +12,12 @@
 * [Programmering i C](http://people.cs.aau.dk/~normark/c-prog-06/pdf/all.pdf) - Kurt Nørmark (PDF)
 
 
-### C Sharp
+### <a name="csharp"></a>C\#
 
 * [Object-oriented Programming in C#](http://people.cs.aau.dk/~normark/oop-csharp/pdf/all.pdf) - Kurt Nørmark (PDF)
 
 
-<h3 id="cpp">C++</h3>
+### <a name="cpp"></a>C++
 
 * [Notes about C++](http://people.cs.aau.dk/~normark/ap/index.html) - Kurt Nørmark (HTML)