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 the banner on docs.parcels-code.org
• Write a release blog post for https://parcels-code.org/blog
• Clean up the migration guide, and put in a nicer format
• Write a page at https://docs.parcels-code.org/en/main/user_guide/examples/explanation_grids.html
• ...