GitBook: [#2] adding back to 2.1 link

Author: jonasjabari

docs/SUMMARY.md

@@ -2,6 +2,7 @@
 
 * [Welcome](README.md)
 * [Migrate to 3.0](migrate-from-2.x-to-3.0.md)
+* [Back to 2.1](https://docs.matestack.io/matestack-ui-bootstrap/v/2.1/)
 
 ## Getting Started
 
@@ -26,22 +27,22 @@
   * [Badge](api/components/badge.md)
   * [Breadcrumb](api/components/breadcrumb.md)
   * [Button](api/components/button.md)
-  * [ButtonGroup](api/components/button_group.md)
+  * [ButtonGroup](api/components/button\_group.md)
   * [Card](api/components/card.md)
   * [Carousel](api/components/carousel.md)
   * [Chart](api/components/chart.md)
   * [Close](api/components/close.md)
   * [Collapse](api/components/collapse.md)
   * [Dropdown](api/components/dropdown.md)
-  * [ListGroup](api/components/list_group.md)
+  * [ListGroup](api/components/list\_group.md)
   * [Modal](api/components/modal.md)
-  * [Nav](api/components/tab_nav.md)
+  * [Nav](api/components/tab\_nav.md)
   * [Navbar](api/components/navbar.md)
   * [Pagination](api/components/pagination.md)
   * [Popover](api/components/popover.md)
   * [Progress](api/components/progress.md)
   * [Scrollspy](api/components/scrollspy.md)
-  * [Smart collection](api/components/smart_collection.md)
+  * [Smart collection](api/components/smart\_collection.md)
   * [Spinner](api/components/spinner.md)
   * [Toast](api/components/toast.md)
   * [Tooltip](api/components/tooltip.md)