Update doc, internal links, layout

• Internal links added between :
  • Reference widget API and
  • solid-display, solid-form, solid-form-search, solid-table pages.
• Internal links added between :
  • concerned attributes in List attributes and
  • solid-form, solid-form-search, widget-mixin, store-mixin, validation-mixin pages.
• Add validation-mixin in index.html.
• Add solid-table attributes in attributes list.
• Layout for paginate-mixin, grouper-mixin.

source/import_documentation/Components/Solid-Display.rst

@@ -55,14 +55,14 @@ Attributes
    a field as it would result in an infinite loop.
 
 ``child-[field]``
-~~~~~~~~~~~~~
-   add attribute ``[field]`` to all children.
+~~~~~~~~~~~~~~~~~
+   Add attribute ``[field]`` to all children.
 
 
 Widgets
 -------
-By default, the widget used is ``<solid-display-div>``. Cf the
-`Widget <https://docs.startinblox.com/import_documentation/Widgets/Reference.html>`__ page for more info.
+   By default, the widget used is ``<solid-display-div>``. Cf the 
+   :ref:`Widget <reference>` page for more info.
 
 
 Events