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

courses/free-courses-it.md

@@ -5,7 +5,7 @@
 * [Architettura degli Elaboratori](#architettura-degli-elaboratori)
 * [Assembly](#assembly)
 * [C](#c)
-* [C Sharp](#c-sharp)
+* [C#](#csharp)
 * [C++](#cpp)
 * [Database](#database)
   * [SQL](#sql)
@@ -66,12 +66,12 @@
 * [Linguaggio C - Corso completo](https://www.youtube.com/playlist?list=PL83Ordjpzm5oUl7tFEjc4iirkPBiv7FxR) - Programmazione Time
 
 
-### C Sharp
+### <a name="csharp"></a>C\#
 
 * [Programmazione ad oggetti in C#](https://www.youtube.com/watch?v=aSgikNnGEKM&list=PLktbfd3yXeH8yQpHM3O468k8l-aTC6Hl6) - G. Pellegrini Parisi
 
 
-<h3 id="cpp">C++</h3>
+### <a name="cpp"></a>C++
 
 * [C++ 11](https://www.youtube.com/playlist?list=PL0qAPtx8YtJfZpJD7uFxAXglkiHSEhktG) (F. Camuso)
 * [C++ libreria QT - playlist 1](https://www.youtube.com/playlist?list=PL0qAPtx8YtJdH4GVwL_3QeJjPcz3DHE2t) (F. Camuso)