Data extensions docs

[adamfri]

No description provided.

[yuvalwix]

Hey Adam, few comments:

• About Extended Fields - i think the developer needs to know that site owners (users) have the ability to add custom fields too. this will be reflected in the API under the _user_defined namespace
• Note: If the object you extended has a revision field, this field is not incremented when you update extended fields using a dedicated endpoint. - this part is a bit unclear. maybe an example can assist?
• To check if there is a dedicated endpoint for updating your extended fields, see the API Reference for the object you are extending. - inaccurate as each entity opted in will have both the unified and dedicated ep
• The maximum storage size for an individual extended field object is 10 KB - the max is per namespace. extended field object may be a little confusing
• worth listing an api get example that app b makes to an entity that app a added fields to it without granting permissions to other apps
• domain event is missing