Proposal to add a "user tutorial"

The current tutorial is great for developers who want to use the GrimoireLab components or engineers who want to install GrimoireLab.

What we are not describing well is how to use GrimoireLab as a user of the GUI. Sure, we can direct people to the Kibana documentation for most parts, but there are GrimoireLab specific modifications and open source metrics specific use cases that we should document.

This issue proposes to add a dedicated user documentation to the GrimoireLab tutorial.

I imagine that in the future the front page of the tutorial offers two sections: user and developer. The current tutorial will be available under the developer section. The new tutorial will be under the user section.

```
GL Tutorial
+ User Doc
+ Dev Doc