[DOCS] Switch seo 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* Remove outdated genindex: not supported anymore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: #103340
Releases: main, 12.4, 11.5
Change-Id: Id690e98fb30595e36eb6d573c7a03098668222dd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83352


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