docs: add Nous Tool Gateway documentation

- New page: user-guide/features/tool-gateway.md covering eligibility,
  setup (hermes model, hermes tools, manual config), how use_gateway
  works, precedence, switching back, status checking, self-hosted
  gateway env vars, and FAQ
- Added to sidebar under Features (top-level, before Core category)
- Cross-references from: overview.md, tools.md, browser.md,
  image-generation.md, tts.md, providers.md, environment-variables.md
- Added Nous Tool Gateway subsection to env vars reference with
  TOOL_GATEWAY_DOMAIN, TOOL_GATEWAY_SCHEME, TOOL_GATEWAY_USER_TOKEN,
  and FIRECRAWL_GATEWAY_URL

website/docs/integrations/providers.md

@@ -49,6 +49,10 @@ The OpenAI Codex provider authenticates via device code (open a URL, enter a cod
 Even when using Nous Portal, Codex, or a custom endpoint, some tools (vision, web summarization, MoA) use a separate "auxiliary" model — by default Gemini Flash via OpenRouter. An `OPENROUTER_API_KEY` enables these tools automatically. You can also configure which model and provider these tools use — see [Auxiliary Models](/docs/user-guide/configuration#auxiliary-models).
 :::
 
+:::tip Nous Tool Gateway
+Paid Nous Portal subscribers also get access to the **[Tool Gateway](/docs/user-guide/features/tool-gateway)** — web search, image generation, TTS, and browser automation routed through your subscription. No extra API keys needed. It's offered automatically during `hermes model` setup, or enable it later with `hermes tools`.
+:::
+
 ### Two Commands for Model Management
 
 Hermes has **two** model commands that serve different purposes: