[DOCS] Add example to RTE configuration concepts

This change adds a real life example to section "Overriding Configuration
via Page TSconfig".

Releases: master,9.5,8.7
Resolves: #90128
Change-Id: Ibb434bc3ec7ff3766186fd26a839fa16cd40ea43
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62928


Tested-by: Felix P. <f.pachowsky@neusta.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Felix P. <f.pachowsky@neusta.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>