[TASK] External manual may replace extension manual

Whenever an external manual is defined and the extension does not
provide an embedded extension manual, link to the external manual
should replace the standard link.

Change-Id: I96ce5074467e07716ddab5c4b249a6b92b085063
Resolves: #61899
Reviewed-on: http://review.typo3.org/32981
Reviewed-by: Tomas Norre Mikkelsen <tomasnorre@gmail.com>
Reviewed-by: Thomas Löffler <loeffler@spooner-web.de>
Tested-by: Thomas Löffler <loeffler@spooner-web.de>