readthedocs: add requirement file. #785

Job	Run time
test-coverage	5m 30s
scanbuild	6m 37s
build-test (ubuntu-20.04, gcc)	4m 33s
coverity-test	0s
build-test (ubuntu-20.04, clang)	4m 21s
test-fuzz	2m 32s
build-test (fedora-32, gcc)	4m 49s
test-mbedtls (ubuntu-20.04)	2m 48s
build-test (fedora-32, clang)	5m 5s
test-no-crypto-build	1m 16s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 41s
build-test (opensuse-leap, gcc)	4m 36s
test-tcti-config	14m 33s
build-test (opensuse-leap, clang)	4m 38s
build-test (ubuntu-22.04, gcc)	4m 39s
build-test (ubuntu-22.04, clang)	4m 40s
build-test (alpine-3.15, gcc)	4m 8s
build-test (alpine-3.15, clang)	4m 48s
	1h 22m 14s

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