docs(voice): add Doubao speech integration examples (TTS + STT)

Author: Hypnus-Yuan

website/docs/user-guide/features/tts.md

@@ -185,6 +185,30 @@ tts:
       output_format: wav
 ```
 
+#### Example: Doubao (Chinese seed-tts-2.0)
+
+For high-quality Chinese TTS via ByteDance's [seed-tts-2.0](https://www.volcengine.com/docs/6561/1257544) bidirectional-streaming API, install the [`doubao-speech`](https://pypi.org/project/doubao-speech/) PyPI package and wire it in as a command provider:
+
+```bash
+pip install doubao-speech
+export VOLCENGINE_APP_ID="your-app-id"
+export VOLCENGINE_ACCESS_TOKEN="your-access-token"
+```
+
+```yaml
+tts:
+  provider: doubao
+  providers:
+    doubao:
+      type: command
+      command: "doubao-speech say --text-file {input_path} --out {output_path}"
+      output_format: mp3
+      max_text_length: 1024
+      timeout: 30
+```
+
+Credentials come from your shell environment (`VOLCENGINE_APP_ID` / `VOLCENGINE_ACCESS_TOKEN`) or `~/.doubao-speech/config.yaml`. Pick a voice by adding `--voice zh-female-warm` (or any other alias from `doubao-speech list-voices`) to the command. `doubao-speech` also bundles streaming ASR — see the [STT section below](#example-doubao--volcengine-asr) for Hermes integration. Source and full docs: [github.com/Hypnus-Yuan/doubao-speech](https://github.com/Hypnus-Yuan/doubao-speech).
+
 #### Placeholders
 
 Your command template can reference these placeholders. Hermes substitutes them at render time and shell-quotes each value for the surrounding context (bare / single-quoted / double-quoted), so paths with spaces and other shell-sensitive characters are safe.
@@ -273,7 +297,25 @@ stt:
 
 **xAI Grok STT** — Requires `XAI_API_KEY`. Posts to `https://api.x.ai/v1/stt` as multipart/form-data. Good choice if you're already using xAI for chat or TTS and want one API key for everything. Auto-detection order puts it after Groq — explicitly set `stt.provider: xai` to force it.
 
-**Custom local CLI fallback** — Set `HERMES_LOCAL_STT_COMMAND` if you want Hermes to call a local transcription command directly. The command template supports `{input_path}`, `{output_dir}`, `{language}`, and `{model}` placeholders.
+**Custom local CLI fallback** — Set `HERMES_LOCAL_STT_COMMAND` if you want Hermes to call a local transcription command directly. The command template supports `{input_path}`, `{output_dir}`, `{language}`, and `{model}` placeholders. Your command must write a `.txt` transcript somewhere under `{output_dir}`.
+
+#### Example: Doubao / Volcengine ASR
+
+If you use [`doubao-speech`](https://pypi.org/project/doubao-speech/) for Doubao TTS (see [above](#example-doubao-chinese-seed-tts-20)), the same package handles speech-to-text via the local-command STT surface:
+
+```bash
+pip install doubao-speech
+export VOLCENGINE_APP_ID="your-app-id"
+export VOLCENGINE_ACCESS_TOKEN="your-access-token"
+export HERMES_LOCAL_STT_COMMAND='doubao-speech transcribe {input_path} --out {output_dir}/transcript.txt'
+```
+
+```yaml
+stt:
+  provider: local_command
+```
+
+Hermes writes the incoming voice message to `{input_path}`, runs the command, and reads the `.txt` file produced under `{output_dir}`. Language is auto-detected by the Volcengine bigmodel endpoint.
 
 ### Fallback Behavior