Bugfix for ROBODoc documentation generation with CMake

Significant change
-------------------------
The `foreach()` loop was overwriting the `ROBO_OUTPUTS` variable instead of adding the directory and concatenating list elements. This resulted in CMake not knowing which files are produced (and need to be removed on `make clean`) and could have led to dependency resolution issues down the road.

minor changes
--------------------
- Missing comma added to README.md
- documentation directory added to the .gitignore list
- More consistent build names and locations compared with source tree
  - CMake now puts built libraries in `lib` subdirectory of the build tree
  - ROBODoc documentation folder renamed from `doc` to `documentation`