[IMP] subscriptions: update sub product doc

[ksc-odoo]

This PR is focused on updating the documentation about Subscription Products to adhere to changes that have occurred on the product forms leading into 18.0

IT SHOULD BE REVIEWED WITH A 17.4 DB (or higher)

It is essentially a top-down overhaul of the previous content to ensure it is as up-to-date as possible.

PROJECT TASK: https://www.odoo.com/odoo/project/3835/tasks/4162186

[robodoo]

[ksc-odoo]

Hey @hojo-odoo -- this one is ready for a Peer Review whenever you get a chance. Thanks! 👍