feat: added the rails engine configuration to the documentation.

Author: Brett Chaldecott

src/content/docs/developer-tools/sdks/backend/ruby-sdk.mdx

@@ -147,6 +147,13 @@ To enable the AuthController, you need to add the following routes to your `conf
     get "client_credentials_auth" => "auth#client_credentials_auth"
   end
 ```
+
+If you decide instead to use the rails engine, you can add the following to your `config/routes.rb` file:
+
+```ruby
+mount KindeSdk::Engine, at: "/kinde_sdk"
+```
+
 ### Usage
 To use the AuthController consume the routes in your application. For example in a Rails view file use the auth path to trigger the login flow: