Updating parcels website and documentation for v4 preview release

[erikvansebille]

This is a Tracking Issue to list all the changes that need to be made on the parcels-code.org and docs.parcels-code.org pages before we can release v4-preview.

• Remove banner on docs.parcels-code.org (Removing v4 warnings in init and docs banner #2835)
• Publish a release blog post for https://parcels-code.org/blog (v4 preview release parcels-code.org#291)
• Clean up the migration guide, and put in a nicer format (Update migration guide #2742)
• Write a page about Grids (Grid explanations user guide (as ipynb) #2786)
• Remove the example_script section ([DOCS] Removing the example scripts from the docs #2749)
• Remove the explicit ds.load() calls in the examples, once raw zarr and/or WindowedArray are implemented (Using to_windowed_arrays() in docs #2748)
• Remove the unstable API warning in __init__ (Removing v4 warnings in init and docs banner #2835)