Migration from `ParcelsRandom` in documentation notebooks

[VeckoTheGecko]

Following from #1618, it would be good to update all the notebooks to use parcels.rng module instead of the ParcelsRandom alias.

i.e.

import parcels
...
def my_kernel(...):
-    parcels.ParcelsRandom ...
+    parcels.rng ...

Support for ParcelsRandom will stay until the next major version, this is just in preparation and to streamline docs.