add AI Hub documentation

- Add 4 new pages: gateway modes concept, configure AI Hub, eject to custom mode, use AI Hub gateway
- Add 2 reusable partials: mode indicator, preference toggles
- Update 5 existing pages with AI Hub mode content
- Document 17 routing rules, 6 backend pools, 6 preference toggles
- Update navigation with new AI Hub pages
- Fix all API paths and ensure docs team standards compliance

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: micheleRP

modules/ROOT/nav.adoc

@@ -45,12 +45,16 @@
 **** xref:ai-agents:mcp/local/configuration.adoc[Configure]
 ** xref:ai-agents:ai-gateway/index.adoc[AI Gateway]
 *** xref:ai-agents:ai-gateway/what-is-ai-gateway.adoc[Overview]
+*** xref:ai-agents:ai-gateway/gateway-modes.adoc[Gateway Modes]
 *** xref:ai-agents:ai-gateway/gateway-quickstart.adoc[Quickstart]
 *** xref:ai-agents:ai-gateway/gateway-architecture.adoc[Architecture]
 *** For Administrators
 **** xref:ai-agents:ai-gateway/admin/setup-guide.adoc[Setup Guide]
+**** xref:ai-agents:ai-gateway/admin/configure-ai-hub.adoc[Configure AI Hub Gateway]
+**** xref:ai-agents:ai-gateway/admin/eject-to-custom-mode.adoc[Eject to Custom Mode]
 *** For Builders
 **** xref:ai-agents:ai-gateway/builders/discover-gateways.adoc[Discover Gateways]
+**** xref:ai-agents:ai-gateway/builders/use-ai-hub-gateway.adoc[Use AI Hub Gateway]
 **** xref:ai-agents:ai-gateway/builders/connect-your-agent.adoc[Connect Your Agent]
 **** xref:ai-agents:ai-gateway/cel-routing-cookbook.adoc[CEL Routing Patterns]
 **** xref:ai-agents:ai-gateway/mcp-aggregation-guide.adoc[MCP Aggregation]