Added auth overview (#731)

Co-authored-by: Sarah <[email protected]>

src/routes/solid-start/advanced/auth.mdx

@@ -0,0 +1,46 @@
+---
+title: "Auth"
+---
+
+Server functions can be used to protect sensitive resources like user data.
+
+```tsx
+"use server"
+
+async function getPrivatePosts() {
+	const user = await getUser()
+	if(!user) {
+		return null  // or throw an error
+	}
+
+	return db.getPosts({ userId: user.id, private: true })
+}
+```
+
+The `getUser` function can be [implemented using sessions](https://docs.solidjs.com/solid-start/advanced/session).
+
+## Protected Routes
+
+Routes can be protected by checking the user or session object during data fetching. 
+This example uses [Solid Router](/solid-router).
+
+```tsx
+const getPrivatePosts = cache(async function() {
+	"use server"
+	const user = await getUser()
+	if(!user) {
+		throw redirect("/login"); 
+	}
+
+	return db.getPosts({ userId: user.id, private: true })
+})
+
+export const route = {
+  load() {
+    void getPrivatePosts()
+  }
+} satisfies RouteDefinition
+```
+
+Once the user hits this route, the router will immediately attempt to fetch `getPrivatePosts` data. 
+If the user is not signed in, `getPrivatePosts` will throw and the router will redirect to the login page.

src/routes/solid-start/advanced/data.json

@@ -4,6 +4,7 @@
 		"middleware.mdx",
 		"session.mdx",
 		"request-events.mdx",
-		"return-responses.mdx"
+		"return-responses.mdx",
+		"auth.mdx"
 	]
 }