!!![WIP]Remove page "Developing a custom ViewHelper"

! redirects must be created
! cross references in other manuals must be fixed

The content was moved to TYPO3 Explained. All cross references
have been adapted.

Related: #536

Documentation/8-Fluid/3-moving-repeating-snippets-to-partials.rst

@@ -71,10 +71,7 @@ ViewHelper namespace import
 Like within Fluid templates, custom ViewHelpers can be used within partials.
 Because these ViewHelpers are not in the default namespace,
 their namespace needs to be imported. For information about how to import a
-namespace, see :ref:`importing-namespaces`.
-
-It's possible to use a :ref:`global-namespace-import`, or :ref:`imports the namespace
-<importing-namespaces>` within the partial, or within the template.
+namespace, see :ref:`t3coreapi:fluid-syntax-viewhelpers-import-namespaces`.
 
 .. note::