[DOCS] Switch scheduler documentation to PHP-based rendering

* Introduce a guides.xml, the presence of this file
  triggers the new rendering.
* Remove outdated genindex: not supported anymore
* Remove old Settings.cfg, was only used for Sphinx

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

Resolves: #103339
Releases: main, 12.4, 11.5
Change-Id: Ifde54f6d21ae29ba8c3e8543601048cab2469744
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83341


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