[DOCS] Link to online manual for beuser,extensionmanager,setup

Resolves: #100457
Releases: main
Change-Id: Ifa947d33af06b92486220b20afb57354870f0393
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78471


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>