How to guide dt and integration scheme

[reint-fischer]

This PR adds a guide describing how to choose an appropriate dt and integration scheme. It is based on @michaeldenes notebook from the 10-years Parcels event.

The basics of how to write a custom kernel are now in the Parcels conceptual workflow explanation, and I have added a bit more text and figures to help users decide what to consider in their simulation setup. @michaeldenes I am mainly interested in your thoughts on how detailed this should be and how to generalize the advice we can give in this guide. Curious to hear what you think!