Bugfix for ROBODoc documentation generation with CMake

Significant change
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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.

The outputs of the ROBODoc generation are now dynamically computed at CMake configure time. (The foreach loop is removed completely.) This generates a list of outputs including full paths.

minor changes
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- 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`