Add multiple languages in the onboarding tutorial

[tbarbette]

It would be great if the "Task tutorial" page would not only show Python but also C, using tabs.

https://github.com/UCL-INGI/INGInious/blob/master/doc/teacher_doc/task_tuto.rst#creating-the-run-file

I guess a common case is to push the whole code of the student into a C file and compile it in run.

Thanks :)