[doc] fix sphinx warnings

5 jobs for master in 10 minutes and 43 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Build
passed #227645
docker
build_linux_37

00:06:16

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passed #227646
docker
build_linux_38

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70.0%
passed #227643
intel macos
build_macos_intel_37

00:09:49

70.0%
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intel macos
build_macos_intel_38

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  Deploy
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docker
deploy_beta

00:00:52