Add extras section

Author: Archmonger

mkdocs.yml

@@ -12,6 +12,7 @@ nav:
                 - Add React to an Existing Project 🚫: learn/add-react-to-an-existing-project.md
                 - Editor Setup: learn/editor-setup.md
                 - React Developer Tools 🚫: learn/react-developer-tools.md
+                - Installable Extras: learn/installable-extras.md
           - More Tutorials:
                 - "Tutorial: React Bootstrap 🚫": learn/tutorial-react-bootstrap.md
                 - "Tutorial: Material UI 🚫": learn/tutorial-material-ui.md