add exercises to week 7

docs/index.md

@@ -27,7 +27,8 @@
   Exercises 15.1, 15.3, 15.4, 16.1, 16.2
 
 * Week 7: [Semiconductors](lecture_7.md)  
-  (based on chapters 17–18)
+  (based on chapters 17–18)  
+  Exercises 17.1–17.4, 18.1, 18.2
 
 * Week 8: Magnetism  
   (based on chapters 19-20)