Docs: backend integration with Guillotina

[stevepiercy]

Document how to integrate Volto with the Guillotina backend.

https://6.docs.plone.org/volto/backend/index.html

[aadityaforwork]

Hello should this documentation be added in docs/backend/index.md or a new readme file is to be created under docs/backend for Guillotina?

[stevepiercy]

Neither. See https://6.docs.plone.org/contributing/documentation/index.html#large-edits and https://6.docs.plone.org/contributing/documentation/index.html#editing-external-package-documentation.