[BUGFIX] Documentation does not initialize signal slot dispatcher

The documentation module does not initialize the signal slot dispatcher
in its ext_localconf.php, but rather reuses the already set variable
from another core extension.

Create the object explicitly before using it.

Resolves: #57009
Releases: 6.2
Change-Id: Ifbd3199f9cef0b2d2d6dacc4569280c5e668822c
Reviewed-on: https://review.typo3.org/28481
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind