[IMP] subscriptions: parent doc restructure

[ksc-odoo]

PROJECT TASK: https://www.odoo.com/odoo/project/3835/tasks/4118666

In an effort to clean up the look (and feel) of the Marketing scope of applications, I am creating this PR to add intro content to the main Subscriptions element on the toctree, add some linked cards

Not only does this help everything appear uniform, but adds a much-needed level of organization and eases the navigability of the documentation. It also sets the table quite nicely for v18.

[robodoo]

[ksc-odoo]

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

[ksc-odoo]

Thanks for the helpful feedback, @dikd-odoo -- I made those adjustments, and believe this one is now ready for Final Review, @StraubCreative -- whenever you get a chance. Thanks! 🙏