DOC: Only recompile Sphinx documentation when required #1224

Job	Run time
cxx-build-workflow / build-test-cxx (ubuntu-22.04)	29m 45s
cxx-build-workflow / build-test-cxx (windows-2022)	32m 23s
cxx-build-workflow / build-test-cxx (macos-13)	29m 24s
cxx-build-workflow / build-test-cxx (macos-15)	16m 30s
python-build-workflow / build-linux-py (11, _2_28-x64)	29m 10s
python-build-workflow / build-macos-arm-py (11)	36m 29s
python-build-workflow / build-linux-py (11, 2014-x64)	29m 50s
python-build-workflow / build-linux-py (11, _2_28-aarch64)	6h 0m 14s
python-build-workflow / build-windows-python-packages (11)	50m 2s
python-build-workflow / publish-python-packages-to-pypi	0s
python-build-workflow / test-linux-notebooks	0s
	10h 13m 47s

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