docs: add several cpp training slides

docs/language/ql-training-rst/query-examples/cpp/data-flow-cpp-1.ql

@@ -0,0 +1,8 @@
+import cpp
+import semmle.code.cpp.commons.Printf
+
+from Call c, FormattingFunction ff, Expr format
+where c.getTarget() = ff and
+      format = c.getArgument(ff.getFormatParameterIndex()) and
+      not format instanceof StringLiteral
+select format, "Non-constant format string."