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.