Create Control Panel needs content

[stevepiercy]

Description

Backend > Control Panels needs content. It will tell developers how to create and deploy a control panel. It should, however, include the process for adding a control panel in Volto, not just the backend. It also needs to be moved into the Developer guide.

Source material:

• https://github.com/plone/training/blob/main/docs/mastering-plone/registry.md
• https://github.com/plone/documentation/blob/5.2-myst/5.2-myst/develop/plone/functionality/controlpanel.md