[CLEANUP] Remove documentation folder from extbase

The present Documentation/README.txt refers to totally outdated
information and contains old links.

The new documentation can be found within the central Core documentation.

Resolves: #61480
Releases: master
Change-Id: I1bf43e0a244fc2e4d45aa5267af4411a5eb713d1
Reviewed-on: http://review.typo3.org/32689
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>