docs: add several cpp training slides

docs/language/ql-training-rst/query-examples/cpp/data-flow-cpp-2.ql

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+import cpp
+import semmle.code.cpp.dataflow.DataFlow
+import semmle.code.cpp.commons.Printf
+
+class SourceNode extends DataFlow::Node { … }
+
+from FormattingFunction f, Call c, SourceNode src, DataFlow::Node arg
+where c.getTarget() = f and
+      arg.asExpr() = c.getArgument(f.getFormatParameterIndex()) and
+      DataFlow::localFlow(src, arg) and
+      not src.asExpr() instanceof StringLiteral
+select arg, "Non-constant format string."