[DOCS] Switch impexp 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
* Remove screenshots.json as the screenshot tool
  is not maintained anymore

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

Resolves: #103302
Releases: main, 12.4, 11.5
Change-Id: Ia79869cdcf9b91c299dfc18dcd1615f5fd3cad3f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83284


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