docs(waiter): Update documentation to use literalinclude directives

- Replaced hardcoded code examples with literalinclude directives
- Set up proper relative paths to reference the test files
- Documentation now automatically reflects the actual test code
- Improved maintainability by keeping code examples in a single location
- Eliminated risk of documentation getting out of sync with code
- Verified documentation builds successfully with included test files

These changes ensure that the documentation will always stay in sync with
the actual implementation in the test files, improving the overall quality
and accuracy of the documentation.

Author: tony

docs/test-helpers/index.md

@@ -10,6 +10,7 @@ environment
 random
 retry
 temporary
+waiter
 ```
 
 ```{eval-rst}