RANG-193: Remove Readspeaker TTS voices from documentation 2022-08-01

[fdisperati-invoca]

Default prompt voices are not indicated. Instead, added a text to explain that defaults are selected based on the campaign country.

RANG-193

PRs for other API versions

• RANG-193: Remove Readspeaker TTS voices from documentation 2022-08-01  #415
• RANG-193: Remove Readspeaker TTS voices from documentation 2022-03-01  #416
• RANG-193: Remove Readspeaker TTS voices from documentation 2021-08-09 #417
• RANG-193: Remove Readspeaker TTS voices from documentation 2020-10-01 #418
• RANG-193: Remove Readspeaker TTS voices from documentation 2019-05-01 #419

Checklist

• Find the Service owning team for these changes, and tag the team as "Reviewers" on this PR
• Test the documentation changes on readthedocs as a private branch
• If changing general content, have agreement on whether to apply to latest version or all versions (if all versions, provide links to the related PRs below)

[Copilot]

Pull request overview

This PR removes deprecated Readspeaker TTS voice options from the API documentation and clarifies how default prompt voices are selected. The changes align with ticket RANG-193 and are part of a coordinated update across multiple API versions.

• Removes two specific voice options (Julie and Bridget) from the English voice options table
• Adds explanatory text indicating that default voices are automatically selected based on campaign country

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[abhig10]