Add Hatstall to tutorial

[GeorgLink]

I was looking for documentation on Hatstall ... I didn't see anything in the tutorial and the search came back empty.

This issue proposes to add a section on Hatstall in the tutorial.

[jsmanrique]

Actually, that would include to update existing HatStall documentation: https://github.com/chaoss/grimoirelab-hatstall/blob/master/README.md https://github.com/chaoss/grimoirelab-hatstall/tree/master/docs