docs(web): add comprehensive Platform Guide documentation

Add new /docs/platform-guide page covering all platform features:
- User accounts: authentication, profiles, usernames
- Collections: creation, tools, MCP integration, env vars
- Agents: configuration, LLM providers, chat interface
- Forking: how to fork agents and collections
- API keys: scopes, rate limits, usage tracking
- Reference: platform limits and URL patterns

apps/web/src/app/docs/page.tsx

@@ -646,6 +646,25 @@ const result = streamText({
               </div>
             </section>
 
+            {/* Platform Guide - Comprehensive reference */}
+            <section className="mb-16 p-6 border border-border rounded-lg bg-surface">
+              <div className="flex items-start gap-4">
+                <span className="text-3xl">📚</span>
+                <div>
+                  <h2 className="text-xl font-bold text-foreground mb-2">Platform Guide</h2>
+                  <p className="text-foreground-secondary mb-4">
+                    Complete guide to TPMJS platform features: user accounts, collections, agents,
+                    forking, and API keys. Everything you need to know in one place.
+                  </p>
+                  <Link href="/docs/platform-guide">
+                    <Button variant="outline" size="sm">
+                      View Platform Guide →
+                    </Button>
+                  </Link>
+                </div>
+              </div>
+            </section>
+
             {/* ==================== API REFERENCE ==================== */}
             <DocSection id="api-overview" title="API Overview">
               <p className="text-foreground-secondary mb-6">

apps/web/src/app/docs/platform-guide/layout.tsx

@@ -0,0 +1,21 @@
+import type { Metadata } from 'next';
+
+export const metadata: Metadata = {
+  title: 'Platform Guide | TPMJS Docs',
+  description:
+    'Complete guide to TPMJS platform features: user accounts, collections, agents, forking, and API keys. Learn how to organize tools, create AI agents, and share your work.',
+  openGraph: {
+    title: 'TPMJS Platform Guide',
+    description:
+      'Complete guide to user accounts, collections, agents, forking, and API keys on TPMJS.',
+    images: [{ url: '/api/og/docs', width: 1200, height: 630 }],
+  },
+  twitter: {
+    card: 'summary_large_image',
+    images: ['/api/og/docs'],
+  },
+};
+
+export default function PlatformGuideLayout({ children }: { children: React.ReactNode }) {
+  return children;
+}