[DOCS] Optimize scheduler documentation

We move chapter on scheduler command line execution to top level in
the menu. This was somewhat hidden under "Installation". Additionally,
we add a link to this chapter to the section on cron.

Additionally the chapter "Installation" is renamed to "Installation &
setup".

"Administration" is renamed to "Backend administration".

Resolves: #90395
Releases: master, 9.5
Change-Id: Id25cdb16f9f3c87c9fdac1194246c5d471d1476c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63271


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>