Incorrect docs for creating a menu item

[tamarasaurus]

The page - https://docs.akeneo.com/latest/design_pim/guides/how_to_customize_menu.html is outdated/incorrect.

1. in the sources, the form extensions declarations are under an extensions: element level
2. the files are located in Resources/config/form_extensions/, not Resources/form_extension/ (under a configdirectory, and plural on extension)
3. if the route name doesn’t start with pim_, the HTTP request is not sent (wut?)
4. root menu elements require a iconModifier config item

cc: @gplanchat