Update to lift test calibration notebook (#1000)

* minor fixes

* shorter forest plots + update first set of saturation curve plots

* jazz up the triangle plot + edit associated text

* add some additional explanations, clarifications

* additional explanation

* add link to quote by Box

* various tweaks and clarifications

* add examples of when lift tests are useful

* tweaks + set one lift test to _decrease_ channel spend

* add brief mathematical description of what add_lift_test_measurements does

* minor fixes

* Adding causal language (#1003)

* Adding causal language

* Rephrasing!

* changes requested by Juanito

---------

Co-authored-by: Juan Orduz <[email protected]>

* single quote -> double quote not picked up by ruff

* update 2nd example in intro

* add contribution plots

---------

Co-authored-by: Carlos Trujillo <[email protected]>
Co-authored-by: Juan Orduz <[email protected]>