docs: mark the public test environment unavailable

Both hostnames the page hands out stopped resolving when the environment
was torn down, so a reader following it meets DNS failures with no
explanation. Say so at the top and point at the production host until
the environment returns.

Signed-off-by: Dmitrii Creed <creeed22@gmail.com>

Author: sorabot

src/getting-started/public-test-environment.md

@@ -1,5 +1,11 @@
 # Public test environment
 
+::: warning NOTE
+
+The public test environment is currently unavailable and the hostnames below do not resolve. Until it is restored, integrate against the production host given in [Calling API endpoints](./calling-api-endpoints.md#base-url).
+
+:::
+
 The test environment is a safe and controlled environment where you can try out and experiment with the FIB API without affecting the production system. Here, you can make mistakes, test different scenarios, and learn how to use the API.
 
 To access the test environment, you need to follow the same steps as for the production environment. The only difference is that you will be provided with test credentials instead of production credentials (see [Connection details](#connection-details) below).