Conflicts:
	src/ngraph/codegen/compiler.cpp

the other backend flag

The CPU backend is disabled by default and must be enabled with
the CMake variable NGRAPH_CPU_ENABLE set to true

Conflicts:
	src/ngraph/runtime/cpu/call_frame.cpp
	src/ngraph/runtime/cpu/call_frame.hpp
	src/ngraph/runtime/cpu/external_function.cpp
	test/cpu.cpp

Fix bug with multiple updates to adjoint map

the LLVM integration requires tar.xz decompression support
which was added in 3.1

Conflicts:
	test/CMakeLists.txt
	test/execute.cpp

Also, add more codegen options

Ops are now being implemented in priority order based on
MNIST MLP requirements

- Adds Sphinx docs support.

- Sphinx and Doxygen docs now built by CI.

- Sphinx and Doxygen docs now installed by `make install`.