Fix sphinx warning

[thomashampson]

Description of work

Sphinx 8.0.2 -> 8.1.0 upgrade found an unused reference, which causes a warning. Then the nighlty builds failed. Here we make use of the reference to fix the warning.

There is no associated issue.

To test:

Ensure the docs build and linux package build pass and that the reference is in a sensible place.

This does not require release notes because it is a minor update to docs

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[robertapplin]

Does the target need to change to release-next?

[SilkeSchomann]

All tests pass and the reference is in a sensible place 👍🏻