[doc] fix sphinx warnings

12 jobs for minimal_load in 3 minutes and 14 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #35320
conda-linux
build_linux_27

00:01:19

passed #35322
conda-linux
build_linux_34

00:01:17

passed #35324
conda-linux
build_linux_35

00:01:16

passed #35325
conda-macosx
build_macosx_27

00:01:59

passed #35323
conda-macosx
build_macosx_34

00:01:54

passed #35321
conda-macosx
build_macosx_35

00:01:55

 
  Test
passed #35330
conda-linux
test_linux_27

00:00:48

90.0%
passed #35328
conda-linux
test_linux_34

00:00:50

90.0%
passed #35326
conda-linux
test_linux_35

00:00:48

90.0%
passed #35327
conda-macosx
test_macosx_27

00:01:10

90.0%
passed #35329
conda-macosx
test_macosx_34

00:01:04

90.0%
passed #35331
conda-macosx
test_macosx_35

00:01:01

90.0%