[DOCS] Switch Low Level documentation to PHP-based rendering

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

Attention: Backports need manual changes to the guides.xml.

Resolves: #103329
Releases: main, 12.4, 11.5
Change-Id: I7ed40a0f5ce1d96003e78ac8860e6dda4c12b225
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83316


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