docs: add Vercel AI Gateway provider guide

[erulkey]

What

Adds Vercel AI Gateway as an LLM provider option in the docs: a new provider guide, a card in the overview, and a nav entry after OpenRouter. Docs only, no code changes.

Why

Gateway exposes an OpenAI-compatible endpoint, so Strix already works with it today through the existing openai/ prefix and LLM_API_BASE (no code change needed) — it just wasn't documented. This gives users a one-endpoint way to reach models across providers with usage and cost tracked in Vercel.

Setup it documents

export STRIX_LLM="openai/anthropic/claude-opus-5"
export LLM_API_KEY="your-ai-gateway-api-key"
export LLM_API_BASE="https://ai-gateway.vercel.sh/v1"

The openai/ prefix selects Strix's OpenAI-compatible client; the rest is the Gateway model ID.

I verified this config end to end against the live Gateway (/v1/models reachable, tool-calling works streaming and non-streaming) before writing the page. The new page mirrors the structure of the existing provider guides (OpenRouter, Anthropic, etc.).

[greptile-apps]

Greptile Summary

Adds documentation for using Vercel AI Gateway through Strix’s existing OpenAI-compatible provider integration.

• Adds a provider guide with environment configuration, model selection, key setup, and benefits.
• Adds the guide to provider navigation and the provider overview.

Confidence Score: 5/5

The documentation-only PR appears safe to merge with no actionable issues identified.

The new route resolves to the added page, its structure follows existing provider documentation, and the documented model, API key, and API base settings align with Strix’s current provider configuration.

Important Files Changed

Filename	Overview
docs/docs.json	Adds a valid navigation entry matching the new provider page route.
docs/llm-providers/overview.mdx	Adds a provider card whose route and MDX structure match existing cards.
docs/llm-providers/vercel-ai-gateway.mdx	Documents a configuration consistent with Strix’s environment-variable handling and OpenAI-compatible model routing.

_{Reviews (1): Last reviewed commit: "docs: add Vercel AI Gateway provider gui..." | Re-trigger Greptile}