Add link to V2 Documentation (#3743)

Added a header to the sidebar to indicate that this is the V1.1.X documentation and added a link to the V2 docs.

docs/_sidebar.md

@@ -1,3 +1,6 @@
+#### V 1.1.X  
+For [V2 docs click here](https://github.com/wix/react-native-navigation/tree/v2)
+
 - Getting started
  - [Installation - iOS](/installation-ios)
  - [Installation - Android](/installation-android)
@@ -14,4 +17,4 @@
   - [Styling the Navigator](/styling-the-navigator)
   - [Adding Buttons to the Navigator](/adding-buttons-to-the-navigator)
   - [Styling the Tab Bar](/styling-the-tab-bar)
-  
+