feat(docs): Add section on client-only routes and auth (#17157)

docs/docs/building-apps-with-gatsby.md

@@ -19,3 +19,7 @@ Some examples of how you could apply this:
 - An e-commerce site with universal product pages and category pages, but also personalized recommendation sections
 
 You can also use your React components to create interactive widgets e.g. allow a user to do searches or submit forms. Because Gatsby is just React, it's easy to blend static and interactive/dynamic models of building web sites.
+
+## Client-only routes & user authentication
+
+Often you want to create a site with client-only routes that are gated by authentication. For more on this approach, check out the reference guide on [client-only routes and authentication](/docs/client-only-routes-and-user-authentication/).