Replace Simple icons with fontawesome icons

I had to change/fix LinkedIn icon to fontawesome one as previous icon :simple-linkedin: wasn’t rendering. 

I noticed the same issue on other Lesson pages. Screenshot below.

<img width="1913" height="1053" alt="Image" src="https://github.com/user-attachments/assets/544dfd1e-88aa-4ec7-b7f4-38a288a8ab28" />