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 Name Job ID Coverage
  Build
manual build:centos6-x86_64 #5790670
cmake build docker linux manual
passed build:debian10-iwyu #5790667
cmake build docker linux

00:01:46

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

00:02:51

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

00:09:37

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