readthedocs: add requirement file. #784

Job	Run time
scanbuild	6m 27s
coverity-test	0s
test-coverage	5m 12s
build-test (ubuntu-20.04, gcc)	4m 30s
test-fuzz	2m 34s
build-test (ubuntu-20.04, clang)	4m 25s
test-mbedtls (ubuntu-20.04)	2m 48s
test-no-crypto-build	1m 28s
build-test (fedora-32, gcc)	4m 43s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 43s
test-tcti-config	14m 4s
build-test (fedora-32, clang)	4m 59s
build-test (opensuse-leap, gcc)	4m 33s
build-test (opensuse-leap, clang)	4m 35s
build-test (ubuntu-22.04, gcc)	4m 27s
build-test (ubuntu-22.04, clang)	4m 32s
build-test (alpine-3.15, gcc)	3m 55s
build-test (alpine-3.15, clang)	4m 46s
	1h 20m 41s

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