docs: add ai-voice-changer playground example

[Dailin521]

Summary

This fixes the missing ai-voice-changer playground route referenced from the AI docs.

Root cause

AI.md referenced the ai-voice-changer playground slug, but the docs playground registry had no matching entry, so the Run button resolved to a 404.

Changes

• add ai-voice-changer example source
• register ai-voice-changer in docs playground examples

Verification

• npm ci
• npm --prefix src/docs run build
• local HTTP check for /AI/
• local HTTP check for /playground/ai-voice-changer/
• local HTTP check for /playground/ai-speech2speech-url/
• local HTTP check for /playground/ai-speech2speech-file/

Closes #2693

[CLAassistant]

All committers have signed the CLA.

[reynaldichernando]

hi @Dailin521 thank you for the PR!
could you help sign the CLA? it's standard for contributions

[Dailin521]

Signed the CLA. Thanks for the heads-up.

[reynaldichernando]

merged! thank you