[BUGFIX] EM: show versions link to read manual

Fixes the "read online" link in extension detail view.

Change-Id: I54cb1c90cb1f11b101a06160a67d2bac34185985
Fixes: #46978
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/19668
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn