docs: address first round of review comments on other slides and add intro to ql decks

docs/language/ql-training-rst/query-examples/cpp/empty-if-cpp-class.ql

@@ -0,0 +1,11 @@
+import cpp
+
+class EmptyBlock extends Block {
+  EmptyBlock() {
+    this.isEmpty()
+  }
+}
+
+from IfStmt ifStmt
+where ifstmt.getThen() instanceof EmptyBlock
+select ifstmt