[TASK] Document the viewhelper directive (#441)

Documentation/WritingReST/Reference/Content/ViewHelper.rst

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+:navigation-title: ViewHelper
+..  include:: /Includes.rst.txt
+..  _viewhelper:
+
+=============================================
+Display Fluid ViewHelpers in ReStructuredText
+=============================================
+
+The special `..  typo3:viewhelper::` directive can be used to display the
+information about a Fluid ViewHelper, including all arguments.
+
+The information on the available arguments is included from a `.json` file.
+See also https://github.com/TYPO3-Documentation/fluid-documentation-generator
+on how to generate the input file.
+
+..  warning::
+    Usage of the viewhelper directive outside of the
+    :ref:`Fluid ViewHelper Reference <t3viewhelper:start>` is still considered
+    experimental.
+
+..  _viewhelper-example:
+
+Example: Display a ViewHelper from a JSON include
+=================================================
+
+..  code-block:: rst
+    :caption: Documentation/MyViewHelper.rst
+
+    ..  typo3:viewhelper:: link.external
+        :source: /resources/global_viewhelpers_demo.json
+
+..  literalinclude:: _ViewHelper/_global_viewhelpers_demo.json
+    :language: json
+    :caption: Documentation/resources/global_viewhelpers_demo.json
+
+
+..  _viewhelper-properties:
+
+Properties of the `viewhelper` directive
+========================================
+
+..  confval-menu::
+    :display: table
+    :type:
+
+    ..  include:: _ViewHelper/_Content.rst.txt
+        :show-buttons:
+
+    ..  include:: _ViewHelper/_Source.rst.txt
+        :show-buttons:
+
+    ..  include:: _ViewHelper/_SortBy.rst.txt
+        :show-buttons:
+
+    ..  include:: _ViewHelper/_NoIndex.rst.txt
+        :show-buttons:

Documentation/WritingReST/Reference/Content/_ViewHelper/_Content.rst.txt

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+..  confval:: [content]
+    :name: directive-viewhelper-content
+    :type: string
+
+    Name of the ViewHelper as used as key in the the JSON include file.

Documentation/WritingReST/Reference/Content/_ViewHelper/_NoIndex.rst.txt

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+..  confval:: :noindex:
+    :name: directive-viewhelper-noindex
+    :type: bool
+
+    If set the ViewHelper will only be displayed but cannot be linked and is not
+    added to any indexes. This is useful if the same ViewHelper should be
+    displayed in several places within the same document. In such a case there
+    would be a clash of identifiers and warnings in the rendering if `:noindex:`
+    is not set.

Documentation/WritingReST/Reference/Content/_ViewHelper/_SortBy.rst.txt

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+..  confval:: :sortBy:
+    :name: directive-viewhelper-sortBy
+    :type: string
+    :default: `name`
+
+    Ordering of the argument list.
+
+    name
+        Sort the arguments alphabetically by name
+
+    json
+        Sort the arguments in the order they appear in the JSON file.
+
+    ..  code-block:: rst
+
+        ..  typo3:viewhelper:: split
+            :source: resources/global_viewhelpers_demo.json
+            :sortBy: name
+
+        ..  typo3:viewhelper:: image
+            :source: resources/global_viewhelpers_demo.json
+            :sortBy: json

Documentation/WritingReST/Reference/Content/_ViewHelper/_Source.rst.txt

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+..  confval:: :source:
+    :name: directive-viewhelper-source
+    :type: string, local file path, relative or absolute
+
+    Path to the source JSON file to be included. Can be relative or absolute.
+
+    ..  code-block:: rst
+
+        ..  viewhelper:: my.viewhelper
+            :source: /some/absolute/path/_includes/_my_viewhelper.json
+
+        ..  viewhelper:: my.viewhelper
+            :source: ../_includes/_my_viewhelper.json
+
+    The path is calculated starting from the :path:`Documentation` directory.
+    It is not possible to include files from different locations in an extension.

Documentation/WritingReST/Reference/Content/_ViewHelper/_global_viewhelpers_demo.json

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+
+{
+  "namespace": "http://typo3.org/ns/TYPO3/CMS/Fluid/ViewHelpers",
+  "viewHelpers": {
+    "split": {
+      "className": "TYPO3Fluid\\Fluid\\ViewHelpers\\SplitViewHelper",
+      "...": "..."
+    },
+    "link.external":{
+      "className": "TYPO3\\CMS\\Fluid\\ViewHelpers\\Link\\ExternalViewHelper",
+      "namespace": "TYPO3\\CMS\\Fluid\\ViewHelpers",
+      "name": "Link\\ExternalViewHelper",
+      "tagName": "link.external",
+      "documentation": "A ViewHelper for creating links to external targets.\n\nExamples\n========\n...",
+      "xmlNamespace": "http://typo3.org/ns/TYPO3/CMS/Fluid/ViewHelpers",
+      "docTags": {},
+      "argumentDefinitions": {
+        "uri": {
+          "name": "uri",
+          "type": "string",
+          "description": "The URI that will be put in the href attribute of the rendered link tag",
+          "required": true,
+          "defaultValue": null,
+          "escape": null
+        }
+      },
+      "allowsArbitraryArguments": true
+    },
+    "...": "..."
+  }
+}