Add chapter or section to admin guide on how to create a new backend add-on and how to include in backend

[ksuess]

Links of affected pages in Plone Documentation, if any.

https://6.docs.plone.org/admin-guide/index.html

Description

https://6.docs.plone.org/admin-guide/index.html is a well improved bundle of chapters, @davisagli Correct me if I am wrong, but I think it's missing a chapter on how to create a new backend add-on and how to include in backend. See https://training.plone.org/mastering-plone/voting-story/index.html#integrate-backend-package-in-training-setup where this is done.