Is there a way to reference an item outside the document directory?

[passeris]

I have a CMake-based way of building our documentation. Basically I have the following structure:

>subsystem_a
- vehicle
  - components
    - componentX  
- proxy
>subsystem_qm
- vehicle
  - components
     - componentA
     - componentB

I am able to build individual documents based on the CMake target selected. All that works fine. The problem is when I need to reference an item from one subsystem to another. For example, suppose componentB has a depends_on relationship with componentX, when I build the document for subsystem_qm, then I will get a warning that item componentX does is not defined. If I include the list of items, then they will be double-defined. Is there a way to somehow import them without duplicating them?

The reason I want to do this is because I want to build a separate document for a certain level of safety and to speed things up when building rather than building a large combined document.

[JasperCraeghs]

External relationships? See #364 (comment)