Redirect to new documentation (#1826)

* Prepare the redirection of this website

* add :slug and versions

* change :slug to :splat

* force redirect and add root redirect

* redirect to contracts

netlify.toml

@@ -1,3 +1,15 @@
 [build]
 command = "npm run docsite build"
 publish = "docsite-build"
+
+[[redirects]]
+  from = "/"
+  to = "https://docs.openzeppelin.com/contracts"
+  status = 301
+  force = true
+  
+[[redirects]]
+  from = "/v2.3.0/*"
+  to = "https://docs.openzeppelin.com/contracts/v2.x/:splat"
+  status = 301
+  force = true