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/',
+                  },
+                },
               ],
             },
             {

pkgs/website/src/content/docs/tutorials/index.mdx

@@ -16,4 +16,9 @@ Learn pgflow through hands-on examples. These tutorials guide you through buildi
     description="Create a workflow that scrapes webpages, analyzes content with OpenAI, and stores results in Postgres"
     href="/tutorials/ai-web-scraper/"
   />
+  <LinkCard
+    title="Use Cases"
+    description="Practical examples for common AI workflows: embeddings, structured output, RAG, and chatbots"
+    href="/tutorials/use-cases/"
+  />
 </CardGrid>