[DOCS] Remove own scheduler task docs in favor of symfony commands

As the preferred and future-proof way of creating commands is using
symfony commands (as a base for scheduler tasks), the documentation
has been adjusted to reflect that, by avoiding promoting the scheduler's
own tasks.

Resolves: #86089
Releases: master
Change-Id: I12e385aaf433ad8caf5d4d43ecf41cf2ff7ceee6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63600


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Daniel Siepmann <coding@daniel-siepmann.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Daniel Siepmann <coding@daniel-siepmann.de>