Write docs

[dpo]

[geoffroyleconte]

Is this issue fixed, or did you mean something more elaborated?

[dpo]

Is it ok if we leave it open? There's almost nothing in the docs. It's not urgent; just so we don't forget.

[geoffroyleconte]

OK, I can add a tutorial

[tmigot]

@geoffroyleconte I noticed you made a tutorial on the jso-docs website https://jso-docs.github.io/introduction-to-quadraticmodels/
Is it possible to make a link to the tutorial in the doc of this package? Thanks!

[geoffroyleconte]

#75