Dev/yangzhang/documents

1. moved the contents of USAGE.txt, README.txt, FunctionList.txt to  the documentations of doxygen 
2. updated the config of doxygen(doxygen.cfg)
3. add notes for doxygen:
     * add mainpage in NE10.h
     * add function description for dsp functions in .c files
     * add function description for math functions in NE10_math.h file
     * add description for structs in NE10_types.h file
     * add description for sample code