docs: add use cases tutorials for AI workflows

Add new tutorial section covering common AI patterns:
- Automatic embeddings with database triggers
- Structured output using Zod schemas
- RAG pipeline with query transformation and reranking
- Chatbot with parallel history and knowledge retrieval

All tutorials demonstrate proper pgflow DSL patterns with
factual, working examples using AI SDK and standard libraries.

Author: claude

pkgs/website/astro.config.mjs

@@ -434,12 +434,24 @@ export default defineConfig({
               link: '/tutorials/',
               id: 'tutorials',
               items: [
+                {
+                  label: 'RAG Pipeline',
+                  autogenerate: {
+                    directory: 'tutorials/rag/',
+                  },
+                },
                 {
                   label: 'AI Web Scraper',
                   autogenerate: {
                     directory: 'tutorials/ai-web-scraper/',
                   },
                 },
+                {
+                  label: 'Use Cases',
+                  autogenerate: {
+                    directory: 'tutorials/use-cases/',
+                  },
+                },
               ],
             },
             {