Update _sidebar.md (#1796)

docs/_sidebar.md

  - [Installation - iOS](/installation-ios)
  - [Installation - Android](/installation-android)
  - [Usage](/usage)
-<!-- 
-- Guide
- - [Top Level API](/top-level-api)
- - [Screen API](/screen-api)
- - [Deep links](/deep-links)
- - [Android Specific Use Cases](/android-specific-use-cases)
- - [Third Party Libraries Support](/third-party-libraries-support)
-
-- Customization
-  - [Styling the Navigator](/styling-the-navigator)
-  - [Adding Buttons to the Navigator](/adding-buttons-to-the-navigator)
-  - [Styling the Tab Bar](/styling-the-tab-bar)
-   -->