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-hi.md

@@ -121,7 +121,7 @@
 * [Web Development Course](https://www.youtube.com/playlist?list=PLfqMhTWNBTe3H6c9OGXb5_6wcc1Mca52n) - Apna College
 
 
-<h4 id="nextjs">Next.js</h4>
+#### Next.js
 
 * [NextJS Tutorial In Hindi](https://www.youtube.com/playlist?list=PLwGdqUZWnOp2rDbpfKAeUi9f8qZMS7_cv) - Vinod Bahadur Thapa (Thapa Technical)