In order to reference to a RST file the usage of a unique document label is the prefered solution.

In our case SDC documentation references documents in the doc project (Service Design User Guide).

Therefor in the referenced documents labels have to be created, which are unique in the namespace.

As naming rule I propose the following:

<project><chapter-x><subchapter-y>_<doc-name>

Example:
doc_user-guides_design_pre-onboarding