Target(s) to generate Doxygen documentation

[ruslo]

Take a look:

• https://github.com/ruslo/sugar/blob/master/cmake/core/sugar_doxygen_generate.cmake
• https://github.com/ruslo/polly/issues/91#issue-165344280
• https://majewsky.wordpress.com/2010/08/14/tip-of-the-day-cmake-and-doxygen/

Uploading:

• https://github.com/miloyip/rapidjson/blob/master/travis-doxygen.sh

[ruslo]

To test: https://github.com/michaeljones/breathe