docs(book): 📝 added hello world and second app tutorials to book

Also deleted everything else in `next`.

Author: arctic-hen7

docs/next/src/SUMMARY.md

@@ -1,23 +1,6 @@
 # Summary
 
 - [Introduction](./intro.md)
-- [Setup](./setup.md)
-- [Architecture](./arch.md)
-- [Building Your First App](./tutorials/first_app/intro.md)
-	- [Installation and Setup](./tutorials/first_app/setup.md)
-	- [Writing Your First Template](./tutorials/first_app/template.md)
-	- [Setting up the App Itself](./tutorials/first_app/app.md)
-- [CLI](./cli.md)
-- [Templates](./templates.md)
-- [Routing](./routing.md)
-- [Error Pages](./error_pages.md)
-- [Rendering Strategies](./strategies/intro.md)
-	- [Build Paths](./strategies/build_paths.md)
-	- [Build State](./strategies/build_state.md)
-	- [Request State](./strategies/request_state.md)
-	- [Revalidation](./strategies/revalidation.md)
-	- [Incremental generation](./strategies/incremental.md)
-- [Building](./building.md)
-- [Serving](./serving.md)
-	- [Actix Web Integration](./integrations/actix-web.md)
-- [Config Managers](./config_managers.md)
+	- [What is Perseus?](./what-is-perseus.md)
+	- [Hello World!](./hello-world.md)
+- [Your Second App](./second-app.md)