Two reasons:
- sphinx requires to have all sources in its source directory
so man pages have to be moved
- layout of subfolders in doc folder was not consistent
- Added make devel target in doc directory.
- Doxygen warnings/errors are now part of the Developer's guide itself.
- renamed mainpage.dox file
- Added extra information to main page
- Extended libdhcp++ descriptions
- Added doc/.gitignore to ignore doc/html (potentially 1000s of files)
