[DOCS] Remove outdated information from RTE documentation

Additionally, fix some flaws and typos.

Resolves: #97760
Releases: main, 11.5
Change-Id: Ic661dec8a180db640a30fb7305b93e6cdb777efe
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74934


Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>