[IMP] Payment providers: advertise and document the "demo" provider

[toaa-odoo]

New page to document the new features of the demo payment provider, and modifications to related pages

task id: 2982367

[robodoo]

Pull request status dashboard.

[toaa-odoo]

Converted to draft because I want your feedback first, since this is a brand new page, and one feature is to come still (currencies).

[toaa-odoo]

Accommodated for changes from https://github.com/odoo/documentation/pull/2775#discussion_r1036132226

[jcs-odoo]

@robodoo r+