Update src/solutions/7_Tight-binding_model_sol.md,...

Update src/solutions/7_Tight-binding_model_sol.md, src/7_tight-binding_model_sol.md, mkdocs.yml files

mkdocs.yml

@@ -34,7 +34,7 @@ nav:
   - Solutions:
     - Einstein model: '1_einstein_model_solutions.md'
     - Debye model: '2_debye_model_solutions.md'
-    - Tight-binding model: '7_Tight-binding_model_sol.md'
+    - Tight-binding model: '7_'
 
 theme:
   name: material

src/7_tight-binding_model_sol.md

+# Solutions for lecture 7 exercises
+
+### Exercise 1: Lattice vibrations
+
+1.
+Hint: Normal modes have the same function as $$\mathbf{e}_1,\mathbf{e}_2,\mathbf{e}_3$$ in $$\mathbb{R}$$.
+Hint: The lectures concerns atom vibrations 

src/solutions/7_Tight-binding_model_sol.md

@@ -4,3 +4,4 @@
 
 1.
 Hint: Normal modes have the same function as $$\mathbf{e}_1,\mathbf{e}_2,\mathbf{e}_3$$ in $$\mathbb{R}$$.
+Hint: The lectures concerns atom vibrations