Add link, redirect for v1 book

This adds a link to the v1 book, and a redirect to a more convinient
alias.

Author: DirectXMan12

docs/book/_redirects

@@ -1,6 +1,9 @@
 https://kubebuilder.netlify.com/* https://book.kubebuilder.io/:splat 301!
 http://kubebuilder.netlify.com/* http://book.kubebuilder.io/:splat 301!
 
+http://v1.book.kubebuilder.io/* https://book-v1.book.kubebuilder.io/:splat 301!
+https://v1.book.kubebuilder.io/* https://book-v1.book.kubebuilder.io/:splat 301!
+
 http://go.kubebuilder.io/* https://go.kubebuilder.io/:splat 301!
 
 https://go.kubebuilder.io/dl/* https://go.kubebuilder.io/releases/:splat 301!

docs/book/src/introduction.md

@@ -1,6 +1,8 @@
 **Note:** Impatient readers may head straight to [Quick
 Start](quick-start.md).
 
+**Using Kubebuilder v1? Check the [legacy documentation](https://v1.book.kubebuilder.io)**
+
 ## Who is this for
 
 #### Users of Kubernetes