build(deps): bump sphinx from 7.4.7 to 8.1.3 in /lib/spack/docs #24

Job	Run time
changes	2s
prechecks / validate	35s
prechecks / style	59s
prechecks / import-check	2m 36s
prechecks / bootstrap-dev-rhel8	3m 59s
prechecks / audit / package-audits (windows-latest, powershell Invoke-Expression -Command "./share/spack/qa/windows_t...	4m 57s
prechecks / audit / package-audits (ubuntu-latest, bash)	8m 9s
prechecks / audit / package-audits (macos-latest, bash)	5m 11s
all-prechecks	1s
bootstrap	0s
unit-tests	0s
coverage	0s
all	0s
	26m 29s

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