Fix Version/s: None
To ensure backwards compatibility during the documenteer 0.6 transition, ensure that the original build-stack-docs CLI can continue to build pipelines.lsst.io without the Doxygen subsite.
Released as Documenteer 0.6.2:
This is consistent with the original behaviour of build-stack-docs, which did not perform a Doxygen build.
This feature is useful for any circumstance when a Doxygen subsite that is normally present is unavailable, such as for a single-package documentation build.