Update Navigation to Include Brand (#474)

Co-authored-by: Joshua Bowden <[email protected]>

Author: JoshBowdenConcepts

theme/src/primer-nav.yml

@@ -20,6 +20,8 @@
       url: https://primer.style/css
     - title: React
       url: https://primer.style/react
+    - title: React Brand
+      url: https://primer.style/brand/
     - title: ViewComponents
       url: https://primer.style/view-components
 - title: Contribute