Review tutorials

The online documentation provides 3 short tutorials: [https://jeka-dev.github.io/jeka/tutorials/basics/](https://jeka-dev.github.io/jeka/tutorials/basics/).
The tutorial MD sources are located in the *docs/tutorials* directory.

To visualize the result, you need to install *mkdocs* as mentioned on the [contributing page](https://github.com/jeka-dev/jeka/blob/master/CONTRIBUTING.md#documentation).

The goal is to validate or improve these tutorials so that they are easily comprehensible for a newcomer.