Collaborative Editing: Move/Link extension author related parts to the extension docs

These helper utilities of the collaborative editing feature in https://directus.io/docs/guides/content/collaborative-editing/development#helper-utilities are relevant and very important for extension authors and should be moved or at least linked to the Interface section: https://directus.io/docs/guides/extensions/app-extensions/interfaces

Also this: https://directus.io/docs/guides/content/collaborative-editing/development#requirements-for-custom-interfaces