Merge branch 'more_docs' into 'NIFTy_8'

.gitignore

.gitlab-ci.yml

demos/custom_nonlinearities.py

demos/getting_started_0.ipynb

demos/getting_started_4_CorrelatedFields.ipynb

docs/generate.sh

docs/source/conf.py

docs/source/user/custom_nonlinearities.ipynb

docs/source/user/getting_started_4_CorrelatedFields.ipynb

docs/source/user/index.rst

src/operators/matrix_product_operator.py

test/test_mpi/test_sync.py