Field guide section on 'variables'

[janvanroestel]

Write a section on what we define as 'variable' nd what is a 'non-variable'

Feature Summary
The main point of this section is to have a clear definition of what we define is a 'variable' and non-variable.

Usage / behavior
same as other pages with a few example lightcurves, but an HR diagram or skylocation might not be relevant here

Implementation details
similar to other pages

Additional context
Add any other context or screenshots about the feature request here.