Sphinx collections cannot be nested

[When I integrate the docs-as-code module in reference-integration](https://github.com/eclipse-score/reference_integration/pull/43), the :docs_combo_experimental build fails:

```
/home/runner/work/reference_integration/reference_integration/docs/_collections/score_docs_as_code/docs/index.rst:6: WARNING: undefined label: '_collections/score_process/process/index' [ref.ref]
```

The problem is the [sphinx-collections](https://sphinx-collections.readthedocs.io/en/latest/) extension which is used in reference-integration and in docs-as-code. When docs-as-code is not the documentation root, it doesn't find the process collection.

Short-term we can simply remove it from docs-as-code since we only really need it reference-integration currently.

Long-term we want a solution which just works for any module which wants to use it. I see two option: Adapt sphinx-collections to handle such a use-case. Stop using sphinx-collections and implement the combo differently.