Merge branch 'fix_docs7' into 'NIFTy_7'

Fix Sphinx warnings

See merge request !572
12 jobs for NIFTy_7 in 28 minutes (queued for 3 seconds)
Status Job ID Name Coverage
  Build Docker
passed #1195633
build_docker_from_scratch

00:14:27

 
  Test
passed #1195639
test_mpi

00:00:21

passed #1195636
test_serial

00:05:35

86.0%
 
  Demo Runs
passed #1195653
run_bernoulli

00:00:28

passed #1195655
run_curve_fitting

00:00:24

passed #1195646
run_getting_started_1

00:00:33

passed #1195648
run_getting_started_2

00:00:59

passed #1195650
run_getting_started_3

00:02:13

passed #1195651
run_getting_started_mf

00:07:56

passed #1195641
run_ipynb0

00:01:33

passed #1195643
run_ipynb1

00:00:41

passed #1195658
run_visual_mgvi

00:01:18