docs(web-search): document xAI Web Search backend (#29052)

Follow-up to #29042 (xAI Web Search provider plugin). Adds xAI to the
canonical user-facing and developer-facing docs, with the search-only
caveat and the LLM-in-a-trench-coat trust model carried over from the
class docstring.

- user-guide/features/web-search.md
  - Backends table: new xAI row + extended search-only note
  - New 'xAI (Grok)' setup section with config knobs and trust-model
    caution admonition
  - Single-backend yaml comment now lists 'xai'
  - Auto-detection table: explicitly note that xAI is NOT auto-detected
    (XAI_API_KEY is shared with inference/TTS/image-gen so we don't
    silently take over web for users who only set it for chat)
- developer-guide/web-search-provider-plugin.md
  - Added plugins/web/xai/ to the 'study these next' reference list
- reference/environment-variables.md
  - XAI_API_KEY description now also mentions web search

website/docs/reference/environment-variables.md

@@ -82,7 +82,7 @@ All variables go in `~/.hermes/.env`. You can also set them with `hermes config
 | `STEPFUN_BASE_URL` | Override StepFun base URL (default: `https://api.stepfun.com/v1`) |
 | `OLLAMA_API_KEY` | Ollama Cloud API key — managed Ollama catalog without local GPU ([ollama.com/settings/keys](https://ollama.com/settings/keys)) |
 | `OLLAMA_BASE_URL` | Override Ollama Cloud base URL (default: `https://ollama.com/v1`) |
-| `XAI_API_KEY` | xAI (Grok) API key for chat + TTS ([console.x.ai](https://console.x.ai/)) |
+| `XAI_API_KEY` | xAI (Grok) API key for chat + TTS + web search ([console.x.ai](https://console.x.ai/)) |
 | `XAI_BASE_URL` | Override xAI base URL (default: `https://api.x.ai/v1`) |
 | `MISTRAL_API_KEY` | Mistral API key for Voxtral TTS and Voxtral STT ([console.mistral.ai](https://console.mistral.ai)) |
 | `AWS_REGION` | AWS region for Bedrock inference (e.g. `us-east-1`, `eu-central-1`). Read by boto3. |