Utilities/Sphinx: Update man page config for Sphinx 4

Sphinx 4 by default generates `man/#/foo.#`, but older versions generate
`man/foo.#` as our install rules expect.  Update our Sphinx config file
to tell Sphinx 4 to use the old layout.

Fixes: #22192
30 jobs for !6126 with sphinx-4 in 9 minutes and 37 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build
manual #5790670
linux docker build cmake manual
build:centos6-x86_64
passed #5790667
linux docker build cmake
build:debian10-iwyu

00:01:46

manual #5790671
linux-3.17 linux docker build cmake manual
build:fedora33-ninja
passed #5790669
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:02:51

passed #5790668
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:09:37

manual #5790673
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-makefiles
manual #5790672
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-ninja
manual #5790674
nonconcurrent msvc-19.28-16.9 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja
 
  Test
created #5790675
x11 linux-3.17 linux docker cmake
test:fedora33-ninja
created #5790677
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-makefiles
created #5790676
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-ninja
created #5790678
nonconcurrent msvc-19.28-16.9 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja
 
  Test Ext
created #5790681
linux docker cuda-rt cmake
test:cuda10.2-nvidia
created #5790679
linux-3.17 linux docker build cmake
test:debian10-ninja
created #5790680
linux-3.17 linux docker build cmake
test:fedora33-makefiles
created #5790682
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi
created #5790683
cmake concurrent xcode-12.4 shell macos
test:macos-x86_64-xcode
created #5790684
concurrent msvc-19.28-16.9 vs2019 shell windows cmake
test:windows-vs2019-x64
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-stager