Add doc for HighFive::DataSpace #793

Job	Run time
Benchmarks	1m 48s
CMake_Project (serial)	2m 2s
CMake_Project (parallel)	1m 40s
Linux_Compilers (clang-10, clang++-10)	3m 25s
Linux_Compilers (clang-12, clang++-12)	3m 21s
Linux_Compilers (gcc-9, g++-9)	5m 54s
Linux_Compilers (gcc-10, g++-10)	3m 46s
Linux_HDF5_Versions (hdf5-1_8_23)	6m 3s
Linux_HDF5_Versions (hdf5-1_10_10)	6m 37s
Linux_HDF5_Versions (hdf5-1_12_2)	6m 52s
Linux_HDF5_Versions (hdf5-1_14_1)	6m 51s
Linux_MPI ({ "os": "ubuntu-20.04", "pkgs": "", "flags": "-DHIGHFIVE_USE_BOOST:Bool=OFF" })	5m 59s
Linux_MPI ({ "os": "ubuntu-20.04", "pkgs": "libboost-all-dev libopencv-dev", "flags": "-DHIGHFIVE_USE_OPENCV:Bool=ON -GNinja" })	6m 9s
Linux_MPI ({ "os": "ubuntu-latest", "pkgs": "libboost-all-dev libeigen3-dev libopencv-dev", "flags": "-DHIGHFIVE_USE_EIGEN:Bool=ON -DHIGHFIVE_USE_OPENCV:Bool=ON -GNinja" })	4m 34s
Linux_MPI ({ "os": "ubuntu-20.04", "pkgs": "libboost-all-dev", "flags": "-DCMAKE_CXX_STANDARD=17" })	6m 37s
Linux_Sanitizers (clang-12, clang++-12, address)	4m 58s
Linux_Sanitizers (clang-12, clang++-12, undefined)	4m 1s
Linux_Sanitizers (gcc-10, g++-10, On)	3m 26s
OSX	3m 17s
Windows (windows-2022, v141, 14)	32s
Windows (windows-2022, v141, 17)	29s
Windows (windows-2022, v143, 14)	11s
Windows (windows-2022, v143, 17)	20s
Windows (windows-2019, v142, 14)	38s
Windows (windows-2019, v142, 17)	2m 53s
	1h 32m 23s

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