docs: add integrations page (#7421)

* add integrations page

* update payment provider docs

* use stripe icon

www/apps/resources/app/integrations/page.mdx

@@ -0,0 +1,31 @@
+import { ChildDocs } from "docs-ui"
+
+export const metadata = {
+  title: `Integrations`,
+}
+
+# {metadata.title}
+
+Find in this document Medusa's modules that integrate third-party services and systems.
+
+## Payment
+
+Learn how to create a payment provider in [this guide](/references/payment/provider).
+
+<ChildDocs showItems={["Payment"]} hideTitle />
+
+---
+
+## Notification
+
+Learn how to create a payment provider in [this guide](/references/notification-provider-module).
+
+<ChildDocs showItems={["Notification"]} hideTitle />
+
+---
+
+## File
+
+Learn how to create a payment provider in [this guide](/references/file-provider-module).
+
+<ChildDocs showItems={["File"]} hideTitle />