DOC: Creating minimal examples and parcels datasets documentation

[VeckoTheGecko]

#1972 added documentation for how we approach test data in Parcels. Let's surface this somewhere in the website (this is probably best to do once we're more sure on a structure for v4 docs)

This shouldn't be copied and pasted, instead should use sphinx to reference the docstring.

[VeckoTheGecko]

This can be put in the contributing page in a section about creating minimal examples