doxygen: adjust documentation directory structure

- Rename documentation/doxygen to documentation/0.doxygen and cleanup
  some unused files.

- Add/rename folders for each sub sections, such as
  1.introduction/...... Each sub section will be created as a subpage.

- Generate initial Doxyfile, this Doxyfile will be used to unify
  doxygen generated API documents and those markdown files under
  documentation folder. This patch just add the default Doxyfile
  generated by running "doxygen -g". It is used as baseline to add
  more features/configurations.

- Rename documentation/README.md to documentation/INDEX.md, and
  use it as mainpage.

- Move 0.doxygen/readme.md to documentation/README.md.

Signed-off-by: Chen Wang <[email protected]>

Author: unicornx