readthedocs: downgrade sphinx #2268

Job	Run time
test-coverage	5m 20s
build-test (ubuntu-20.04, gcc)	4m 29s
coverity-test	0s
scanbuild	6m 46s
build-test (ubuntu-20.04, clang)	4m 25s
test-fuzz	2m 33s
build-test (fedora-32, gcc)	4m 43s
test-mbedtls (ubuntu-20.04)	2m 46s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 39s
test-no-crypto-build	1m 32s
test-tcti-config	13m 55s
build-test (fedora-32, clang)	4m 50s
build-test (opensuse-leap, gcc)	4m 30s
build-test (opensuse-leap, clang)	4m 38s
build-test (ubuntu-22.04, gcc)	4m 32s
build-test (ubuntu-22.04, clang)	4m 37s
build-test (alpine-3.15, gcc)	3m 56s
build-test (alpine-3.15, clang)	4m 42s
	1h 20m 53s

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