Add missing import statements in generated docs

[lucassus]

This pull request updates the documentation to include necessary import statements for initializing the AdvancedBillingClient and setting up Basic Authentication credentials. These changes aim to improve clarity and usability for developers referencing the documentation.

Documentation improvements:

• README.md: Added import statements for AdvancedBillingClient, Environment, and BasicAuthCredentials in the example code for initializing the API client.
• doc/auth/basic-authentication.md: Included import statements for AdvancedBillingClient and BasicAuthCredentials in the example code for setting up Basic Authentication.
• doc/client.md: Updated the example code for initializing the API client to include import statements for AdvancedBillingClient, Environment, and BasicAuthCredentials.

[mykhi]

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