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/sidebars.ts

@@ -36,6 +36,7 @@ const sidebars: SidebarsConfig = {
       collapsed: true,
       items: [
         'user-guide/features/overview',
+        'user-guide/features/tool-gateway',
         {
           type: 'category',
           label: 'Core',