📝 (admin) add admin docs

docs/pages/2.usage/5.admin.md

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+# Admin
+
+:::alert{type="info"}
+Admin is currently under development. Informations below are related to developer experience for the Admin UI.
+:::
+
+Admin application is a **Vue 3** app built aside from the current app.
+
+This app exposes an **HTTP API** allowing to perform **read/write** operations on the current project files and directories.
+
+The Docs app is rendered via an `iframe`, content updates are so reflected using **Docus core** and **WebSockets**.
+
+## Start the UI
+
+The admin UI can be launched without the Docs repository, using a command.
+
+```bash
+yarn dev:admin
+```