add a video link

src/9_crystal_structure.md

@@ -49,6 +49,10 @@ _based on chapter 12 of the book_
     - Determine the primitive, conventional, and Wigner-Seitz unit cells of a given lattice
     - Determine the Miller planes of a given lattice
 
+??? info "Lecture video"
+
+    <iframe width="100%" height="315" src="https://www.youtube-nocookie.com/embed/zl8htOplU1s" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+
 ### Crystal classification
 
 ```python