[DOCS] Create basic documentation for Reactions

This is mainly the scaffolding for future more extensive documentation.
Based on the Changelog entry.

Resolves https://github.com/TYPO3-Documentation/Changelog-To-Doc/issues/280

Resolves: #99251
Releases: main
Change-Id: I0ce1bfb575f13bcfffb6bf2d67c8dad1c63a907f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76887


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Chris Müller <typo3@krue.ml>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Chris Müller <typo3@krue.ml>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>