Add fesom tutorial to documentation

[fluidnumericsJoe]

Description

This tutorial uses the provided fesom2 channel data for demonstrating the basics of getting started with FESOM2 (unstructured grid) in parcels

Checklist

• [ X ] Closes #xxxx
• [ X ] This PR targets the correct branch (main for normal development, v3-support for v3 support)

[erikvansebille]

Looks great; two small comments below

[erikvansebille]

As I can't easily comment on specific lines, here two comments:

1. In other tutorials, we always use ds = parcels.convert.fesom_to_ugrid(ds) (instead of ds = fesom_to_ugrid(ds)) to make explicit that these convert functions are part of parcels. Do that here too?
2. Is there an easy/quick way to also plot the velocity field before the pset.execute()? In that way, users get a feel for what kind of flow to expect. And that's a nice help for them to visualise their own unstructured flow field