fix: add sphinx_rtd_theme to readthedocs yml #192

Job	Run time
Build wheels on macos-latest / x86_64	2m 9s
Build wheels on macos-latest / arm64	2m 9s
Build source distribution	54s
Build wheels on ubuntu-latest / x86_64	2m 8s
upload_pypi	0s
	7m 20s

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