Write develop docs about extrapolation

[shellmayr]

Extrapolation will necessitate some changes to the product surface, including e.g. alerts, equations, trace explorer, and more. For all of these parts, a smart way of handling the wanted and unwanted effects of extrapolation is needed to provide the most user value. Therefore, for any developer that will work on product surface that needs to integrate some logic around extrapolation, we should provide documentation about what extrapolation entails, how it affects different types of data (e.g. counts vs numeric), and how to best make product decisions in this context.