Add build instructions for Huawei's HarmonyOS to the documentation #5245

Job	Run time
build (ubuntu-latest, gfortran, cmake)	41m 51s
msys2 (UCRT64, int32, Release)	18m 26s
build (ubuntu-latest, gfortran, make)	20m 10s
msys2 (UCRT64, int64, Release)	16m 31s
build (ubuntu-latest, flang, cmake)	43m 50s
msys2 (MINGW32, int32, Release)	18m 54s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	5m 41s
build (ubuntu-latest, flang, make)	18m 56s
msys2 (CLANG64, int32, Release)	16m 42s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	9m 53s
build (macos-latest, gfortran, cmake)	13m 54s
msys2 (CLANG64, int64, Release)	18m 20s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	5m 36s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	15m 44s
build (macos-latest, gfortran, make)	5m 52s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	4m 48s
	4h 35m 8s

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