[DOCS] Update ext:rte_ckeditor documentation

With the update from CKEditor 4 to CKEditor 5, parts of the
extension documentation got outdated.

This patch includes the following changes:

* Updated external links to CKEditor 5 documentation
* Updated screenshots to show CKEditor 5
* Updated configuration examples and removed unsupported options
* Updated configuration reference
* Added "Grouping toolbar items in drop-downs" example
* Added "How do I create a custom plugin?" example

Resolves: #101465
Releases: main, 12.4
Signed-off-by: Torben Hansen <derhansen@gmail.com>
Change-Id: Iefa020a577ab3ab03f95ba8584607196f69b8886
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80479


Tested-by: core-ci <typo3@b13.com>
Tested-by: Lina Wolf <112@linawolf.de>
Reviewed-by: Lina Wolf <112@linawolf.de>