The reason url field is not available in the list of available lookup fields is that Fibery does not support collection of primitive fields at the moment.
For example you cannot create a field that will be an array of text values. You need to create several text fields instead. But in case of relation fields we do support collections. For example you can create Bugs relation field for UserStory type. And setup relation arity as one-to-many. So you can relate several bugs to one userstory.
In your case as Reference Docs field is a collection, so the result lookup field will be a collection itself. That is why only relations will be available in Field’s list.
As a workaround you may create a formula field like this:
[Reference Docs].Join([your url field], ",")
This will be a text field with all urls from related Reference Docs, separated by comma.