Add sample smart cell livebook to test iframes

data/smart_cell.livemd

@@ -0,0 +1,15 @@
+# SmartCell
+
+```elixir
+Mix.install([
+  {:kino_db, "~> 0.1.3"},
+  {:postgrex, "~> 0.16.3"}
+])
+```
+
+## Postgres
+
+<!-- livebook:{"attrs":{"type":"postgres","port":5432,"variable":"conn"},"kind":"Elixir.KinoDB.ConnectionCell","livebook_object":"smart_cell"} -->
+
+```elixir
+```