Docs: stand up the GemStack documentation site (VitePress)

[suleimansh]

Why

GemStack is positioned as standalone, community-governed infrastructure, but it has no documentation home of its own. Today:

• Each package ships only a short quickstart README (54-189 lines).
• The deep, narrative documentation for this engine still lives in the rudder repo (docs/guide/ai.md, 1090 lines; docs/guide/mcp.md, 229 lines), framed entirely around @rudderjs/ai / @rudderjs/mcp (the old scope, before the engine graduated to @gemstack/*).

So the docs that describe GemStack's engine sit in another repo under the wrong scope, and GemStack itself has no guide or site.

What

Stand up a VitePress docs site in docs/ (mirroring the stack GemStack already knows from rudder, deployable to gemstack.land) that owns the framework-agnostic engine documentation. Migrate and neutralize the engine content from rudder's ai.md / mcp.md into per-package guides written for @gemstack/* with zero rudder framing. Document only what the @gemstack packages actually export; anything that is a Rudder binding (ORM-backed stores, the /server provider, make:agent / ai:eval CLI) stays out, noted as living in @rudderjs/ai.

Scope

• VitePress scaffold: docs/ workspace package, .vitepress/config.ts, theme, landing page
• guide/ overview + installation/quickstart
• packages/ai-sdk/* deep guides (agents, tools, streaming, agents/sub-agents, structured output, memory, RAG, testing/evals, providers)
• packages/ai-skills guide
• packages/ai-autopilot guide
• packages/ai-mcp guide
• packages/mcp guide
• docs:build is green (no dead links)

Follow-up (separate rudder PR)

Once this lands, trim rudder's ai.md / mcp.md to the binding surface (configureAiQueue, container/telescope integration) and link out to the GemStack site for the engine.