docs(ai): add text-to-speech docs (#679)

* docs(ai): add text-to-speech docs

* refactor: revert openapi.yml file change

This commit reverts the openapi.yaml file change that was done since
this file is automatically updated using the Speakeasy Github action.

* refactor(ai): ensure TTS example command works

This commit ensure that the example command in the TTS pipeline page can
be directly used in the terminal without having to escape the '.

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Co-authored-by: Rick Staa <[email protected]>

Author: pschroedl