📝 docs: Add documentation

docs/api/reference.md

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+---
+title: API Reference
+outline: false
+aside: false
+---
+
+# API Reference
+
+The API reference now uses Scalar (embedded via iframe) to render the OpenAPI files hosted in [`public/swagger`](https://github.com/perfect-panel/ppanel-docs/tree/main/public/swagger). Each schema has its own dedicated page:
+
+- [Common Service](./common) — shared authentication/utilities used by both user and admin portals.
+- [User Service](./user) — subscriptions, orders, tickets, profile management.
+- [Admin Service](./admin) — dashboard, maintenance, commerce, logging modules.
+- [Gateway](./gateway) — version checks, heartbeats, registration, and update orchestration hooks for edge services.
+
+Pick a service from the sidebar to view the interactive API explorer.