[DOCS] Switch Redirects 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* Remove outdated genindex: not supported anymore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: #103336
Releases: main, 12.4, 11.5
Change-Id: I31f40444705530a3a9219fde6530cc46f3888df7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83345


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