TravisCI: Incorporate documentation testing
TravisCI: Incorporate documentation testing

- Run sphinx-build on the docs/ directory, and ensure that the documentation compiles without any warnings or errors.
- Run generate_user_docs.sh, and fail testing if the generated documentation files are not identical (i.e. the developer has forgotten to run this script, and hence a mismatch remains between the command-based help pages and the documentation reference pages).

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3