RANG-193: Remove Readspeaker TTS voices from documentation 2021-08-09

[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)

[abhig10]

[abhig10]

The 2021-08-09 version was created but it was never publically released. I still favor merging this branch for consistency purpose. So you could just merge this branch and not worry about releasing it.

[abhig10]

You can probabaly close this PR because this version is very much behind other versions https://github.com/Invoca/developer-docs/tree/2021-08-09/source/api_documentation/network_integration and will not be released.

[fdisperati-invoca]

You can probabaly close this PR because this version is very much behind other versions https://github.com/Invoca/developer-docs/tree/2021-08-09/source/api_documentation/network_integration and will not be released.

If that's the case, then we should just delete branch 2021-08-09. If we can't do that yet, then I'd merge this PR to keep things consistent.