[TASK] Add missing Documentation/9.0/Index.rst

Due to the missing index file, the 9.0 doc files
are not yet rendered on docs.typo3.org

Change-Id: I65781b9de84dd4a0f1205ded175858a90dc01f85
Resolves: #83304
Releases: master
Reviewed-on: https://review.typo3.org/55056


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>