readthedocs: Use dirhtml builder. #1921

Job	Run time
osx clang --disable-ssl	3m 46s
dpdk gcc	7s
linux rpm fedora	7m 26s
linux deb no dpdk	7m 4s
linux gcc --disable-ssl	3m 32s
linux clang --disable-ssl	3m 12s
linux gcc c99	3m 21s
linux clang c99	3m 40s
linux gcc test	11m 3s
linux clang test asan	20m 33s
linux clang test ubsan	12m 42s
linux gcc test --enable-shared	17m 24s
linux clang test --enable-shared	17m 48s
linux gcc test dpdk	10m 30s
linux clang test dpdk	11m 15s
linux gcc test -ljemalloc	11m 15s
linux clang test -ljemalloc	11m 29s
linux gcc --enable-afxdp	3m 19s
linux clang --enable-afxdp	3m 0s
linux gcc dpdk --enable-shared	4m 55s
linux clang dpdk --enable-shared	6m 0s
linux gcc dpdk-shared	4m 38s
linux clang dpdk-shared	4m 13s
linux gcc dpdk-shared --enable-shared	4m 52s
linux clang dpdk-shared --enable-shared	4m 16s
linux gcc m32 --disable-ssl	3m 16s
	3h 14m 36s

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