[BUGFIX] Link to the advanced system requirements at TYPO3 Explained (#656)

Releases: main, 13.4, 12.4

(cherry picked from commit e0288ad)

Author: linawolf

Documentation/Installation/SystemRequirements/Index.rst

@@ -12,3 +12,7 @@ You will need a webserver, an up-to-date version of PHP and an SQL database. See
 here which versions of TYPO3 are current and their requirements
 `get.typo3.org <https://get.typo3.org/version/#system-requirements>`_.
 
+..  seealso::
+    Advanced information on which PHP configuration and extensions are required and suggested,
+    how to configure the webserver and database etc can be found in
+    `TYPO3 Explained, System Requirements <https://docs.typo3.org/permalink/t3coreapi:system-requirements>`_.