Add a reentrancy vulnerability example

[annenkov]

Task description

Create a folder named something like vulnerability-examples and add the complete code of the DAO example https://github.com/Concordium/concordium.github.io/pull/722

[abizjak]

@annenkov Do you still plan to do anything with this?

[annenkov]

@abizjak I think it still makes sense to add. I have this example, but the testing approach has changed, so I think it should be changed first to use the integration testing library. The documentation should be updated too.