add Chapter 5-1.

add Chapter number in Doxygen comment.

fix include guard of `src/chapter_02.h`.

remove unnecessary enum.

add: Chapter 2-6 and 2-11.

add: Chapter 2-4 (absolute value) and 2-5 (average).

update: fix doxygen comment.

update: fix `#include` as renamed header file name.

renamed

update: some bit manipulating functions in chapter 2-1.

add: turn on the right most 0-bit described in Chapter 2-1.

update: add Doxyfile and fix the header comment to output doc properly

add: Chapter 2-1 sources and tests

Initial commit