[ai-document-translation] Readme code snippets have missing imports

Update the code snippets on readme and make sure they have all the required imports. For instance, https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/translation/azure-ai-translation-document/README.md#create-the-client-with-azurekeycredential has missing `os import` .
If we want to make this compatible with AI agents and azure-skills-python, the readmes is the place the agents and skills get their context from and get regenrated. So make sure they are in the correct shape.