docs: Add some docs for unstable middleware (#13930)

Author: brophdawg11

docs/api/utils/createContext.md

@@ -0,0 +1,80 @@
+---
+title: createContext
+unstable: true
+---
+
+# unstable_createContext
+
+[MODES: framework, data]
+
+<br/>
+<br/>
+
+<docs-warning>This API is experimental and subject to breaking changes. Enable it with the `future.unstable_middleware` flag.</docs-warning>
+
+## Summary
+
+[Reference Documentation ↗](https://api.reactrouter.com/v7/functions/react_router.unstable_createContext.html)
+
+Creates a type-safe context object that can be used to store and retrieve values in middleware, loaders, and actions. Similar to React's `createContext`, but designed for React Router's request/response lifecycle.
+
+## Signature
+
+```tsx
+unstable_createContext<T>(defaultValue?: T): RouterContext<T>
+```
+
+## Params
+
+### defaultValue
+
+An optional default value for the context. This value will be returned if no value has been set for this context.
+
+## Returns
+
+A `RouterContext<T>` object that can be used with `context.get()` and `context.set()` in middleware, loaders, and actions.
+
+## Examples
+
+### Basic Usage
+
+```tsx filename=app/context.ts
+import { unstable_createContext } from "react-router";
+
+// Create a context for user data
+export const userContext =
+  unstable_createContext<User | null>(null);
+```
+
+```tsx filename=app/middleware/auth.ts
+import { userContext } from "~/context";
+import { getUserFromSession } from "~/auth.server";
+
+export const authMiddleware = async ({
+  request,
+  context,
+}) => {
+  const user = await getUserFromSession(request);
+  context.set(userContext, user);
+};
+```
+
+```tsx filename=app/routes/profile.tsx
+import { userContext } from "~/context";
+
+export async function loader({
+  context,
+}: Route.LoaderArgs) {
+  const user = context.get(userContext);
+
+  if (!user) {
+    throw new Response("Unauthorized", { status: 401 });
+  }
+
+  return { user };
+}
+```
+
+## See Also
+
+- [Middleware Guide](../../how-to/middleware)