#1682 - Fixing form_extensions and slightly improving the guide

[Dnd-Smash]

Description

As explained in the issue #1682, some form extensions contents do not work with version 5.0

Definition Of Done

• used the csv_product_import_(show/edit).yml as starting point and refined them a bit to match this guide's need.
• added a bit of explanation before the raw form extensions contents in the guide to make it a bit nicer

[nestorAkeneo]

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