[BUGFIX] Broken icons in extension manager

fix broken icons in extension manager from
EXT:documentation.
Documentation: added icons to button group.

Resolves: #63430
Releases: master
Change-Id: I753685acb93ca29662c2e437b2332b18637698ca
Reviewed-on: http://review.typo3.org/34779
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>