Merge branch 'doc_fix' into 'NIFTy_6'

Fix Sphinx warnings

See merge request !573
14 jobs for NIFTy_6 in 25 minutes and 50 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build Docker
passed #1195631
build_docker_from_scratch

00:12:32

 
  Test
passed #1195637
test_mpi

00:00:37

passed #1195634
test_serial

00:04:25

86.0%
 
  Release
passed #1195640
pages

00:01:07

 
  Demo Runs
passed #1195654
run_bernoulli

00:00:36

passed #1195656
run_curve_fitting

00:00:33

passed #1195645
run_getting_started_1

00:00:39

passed #1195647
run_getting_started_2

00:01:00

passed #1195649
run_getting_started_3

00:02:48

passed #1195652
run_getting_started_mf

00:07:43

passed #1195642
run_ipynb0

00:01:28

passed #1195644
run_ipynb1

00:00:41

passed #1195657
run_visual_mgvi

00:01:17

 
  Deploy
passed pages:deploy

00:00:02