Third-party dependences for Graal

As stated in https://lists.linuxfoundation.org/pipermail/grimoirelab-discussions/2020-January/000282.html, the [intro](https://chaoss.github.io/grimoirelab-tutorial/graal/intro.html) requires Python3 and Graal to be installed, but it does not mention that **third-party backend-tools are also needed**.

Following the tutorial lacking them will cause errors.

- Just mentioning this in the intro would already help a lot to interprete the error messages.
- Adding a guide on how to install theese would help further to solve the problem. https://github.com/chaoss/grimoirelab/tree/master/third-party#example is already ok for Docker users or to find out that the Docker [grimoirelab/full](https://hub.docker.com/r/grimoirelab/full) image provides 2 deb files to install these in Debian. A generic description would be even better.
- Capturing theese kind of errors in [Graal](https://github.com/chaoss/grimoirelab-graal) itself and pointing the user to the previous documentation would be the next step. This would possibly require a sepparate issue (?).