Add example of simple environment config extending

Follow up to rails#47570

[ci skip]

Update guides/source/configuring.md

Co-authored-by: Hartley McGuire <[email protected]>

Update guides/source/configuring.md

Co-authored-by: Jonathan Hefner <[email protected]>

Author: morgoth

guides/source/configuring.md

@@ -3018,7 +3018,16 @@ development:
 
 By default Rails ships with three environments: "development", "test", and "production". While these are sufficient for most use cases, there are circumstances when you want more environments.
 
-Imagine you have a server which mirrors the production environment but is only used for testing. Such a server is commonly called a "staging server". To define an environment called "staging" for this server, just create a file called `config/environments/staging.rb`. Please use the contents of any existing file in `config/environments` as a starting point and make the necessary changes from there.
+Imagine you have a server which mirrors the production environment but is only used for testing. Such a server is commonly called a "staging server". To define an environment called "staging" for this server, just create a file called `config/environments/staging.rb`. Since this is a production-like environment, you could copy the contents of `config/environments/production.rb` as a starting point and make the necessary changes from there. It's also possible to require and extend other environment configurations like this:
+
+```ruby
+# config/environments/staging.rb
+require_relative "production"
+
+Rails.application.configure do
+  # Staging overrides
+end
+```
 
 That environment is no different than the default ones, start a server with `bin/rails server -e staging`, a console with `bin/rails console -e staging`, `Rails.env.staging?` works, etc.