[DOCS] Switch styleguide 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* Remove outdated genindex: not supported anymore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: #103341
Releases: main
Change-Id: Ie7eea104f996882b07017e6965a95cd8aa966a5f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83353


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