docs(tts): mention xAI custom voice support (#18776)

Point users to xAI's custom voices feature — clone your voice in the
console, paste the voice_id into tts.xai.voice_id. No code changes
needed; the existing TTS pipeline already handles arbitrary voice IDs.

- config.py: link to xAI custom voices docs in voice_id comment
- setup.py: prompt accepts custom voice IDs during xAI TTS setup
- tts.md: short section linking to xAI console and docs

hermes_cli/config.py

@@ -830,7 +830,7 @@ DEFAULT_CONFIG = {
             # Voices: alloy, echo, fable, onyx, nova, shimmer
         },
         "xai": {
-            "voice_id": "eve",
+            "voice_id": "eve",  # or custom voice ID — see https://docs.x.ai/developers/model-capabilities/audio/custom-voices
             "language": "en",
             "sample_rate": 24000,
             "bit_rate": 128000,