build(deps): bump sphinx-rtd-theme from 2.0.0 to 3.0.1 in /lib/spack/docs #733

Job	Run time
changes	3s
prechecks / validate	35s
prechecks / style	56s
prechecks / bootstrap-dev-rhel8	4m 33s
prechecks / import-check	2m 52s
prechecks / audit / package-audits (windows-latest, powershell Invoke-Expression -Command "./share/spack/qa/windows_t...	5m 8s
prechecks / audit / package-audits (ubuntu-latest, bash)	8m 35s
prechecks / audit / package-audits (macos-latest, bash)	5m 42s
all-prechecks	1s
bootstrap	0s
unit-tests	0s
coverage	0s
all	0s
	28m 25s

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