[DOCS] Switch Reactions documentation to PHP-based rendering

* Introduce a guides.xml, the presence of this file
  triggers the new rendering.
* Remove old Settings.cfg, was only used for Sphinx

Attention: Backport needs manual changes to the guides.xml.

This manual exists since TYPO3 v12

Resolves: #103332
Releases: main, 12.4
Change-Id: Ief1b99941b36f76189281423caa9f5badd292711
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83323


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