Website: Add nice redirects for the new docs site (#1681)

This the following redirections as per @juanmaguitar request:

* from https://playground.wordpress.net/builder to
https://playground.wordpress.net/builder/builder.html
* from https://playground.wordpress.net/wordpress to
https://playground.wordpress.net/wordpress.html
* from https://playground.wordpress.net/gutenberg to
https://playground.wordpress.net/gutenberg.html
* from https://playground.wordpress.net/proxy to
https://github-proxy.com/

In this way it would become easier to access (and remember) some of the
tools related to Playground:

* https://playground.wordpress.net/ → Playground instance
* https://playground.wordpress.net /docs → Playground Docs (redirection
already active)
* https://playground.wordpress.net /builder → Playground Blueprints
Builder
* https://playground.wordpress.net /wordpress → Playground PR viewer for
WordPress
* https://playground.wordpress.net /gutenberg → Playground PR viewer for
Gutenberg
* https://playground.wordpress.net /proxy  → Playground Proxy Service

 ## Testing instructions

Unfortunately the only way to test is by merging and deploying the
website.

packages/playground/website-deployment/custom-redirects-lib.php

@@ -188,6 +188,34 @@ function playground_maybe_redirect( $requested_path ) {
 		);
 	}
 
+	if ( str_ends_with( $requested_path, '/builder' ) ) {
+		return array(
+			'location' => 'https://playground.wordpress.net/builder/builder.html',
+			'status' => 301
+		);
+	}
+
+	if ( str_ends_with( $requested_path, '/wordpress' ) ) {
+		return array(
+			'location' => 'https://playground.wordpress.net/wordpress.html',
+			'status' => 301
+		);
+	}
+
+	if ( str_ends_with( $requested_path, '/gutenberg' ) ) {
+		return array(
+			'location' => 'https://playground.wordpress.net/gutenberg.html',
+			'status' => 301
+		);
+	}
+
+	if ( str_ends_with( $requested_path, '/proxy' ) ) {
+		return array(
+			'location' => 'https://github-proxy.com/',
+			'status' => 301
+		);
+	}
+
 	if ( str_ends_with( $requested_path, '/wordpress-browser.html' ) ) {
 		return array(
 			'location' => '/',