[ak_aiyagari] Add source file

Author: shizejin

lectures/_toc.yml

@@ -79,6 +79,7 @@ parts:
   - file: cass_fiscal
   - file: cass_fiscal_2
   - file: ak2
+  - file: ak_aiyagari
   - file: cake_eating_problem
   - file: cake_eating_numerical
   - file: optgrowth