Add docs on TreeView checkboxes (#206)

* chore(treeview-checkboxes): inital structure and single node article

* chore(treeview-checkboxes): overview article

* chore(checkboxes-treeview): overview article

* chore(events): add checkeditemschanged event to the events article

* chore(treeview): add example image

* chore(treeview-checkboxes): add the checkboxes folder to the config.yml file

* chore(treeview): minor improvements

* chore(treeview-checkboxes): move the checking and selecting node example to the overview article

Co-authored-by: Marin Bratanov <m.bratanov@gmail.com>

Author: svdimitr

_config.yml

@@ -131,6 +131,9 @@ navigation:
      "*components/treeview/selection":
         title: "Selection"
         position: 3
+     "*components/treeview/checkboxes":
+        title: "CheckBoxes"
+        position: 4
      "*components/contextmenu/data-binding":
         title: "Data Binding"
         position: 2