Add demo mode docs

Author: Mozzo1000

docs/docs/04 - Features/Demo-mode.md

@@ -0,0 +1,21 @@
+# Demo Mode
+BookLogr includes a demo mode feature for showcasing the applications functionality. This mode is used for the official demo site at https://demo.booklogr.app.cd
+
+To try the official demo website, use the following login credentials:
+- **Username**: `demo@booklogr.app`
+- **Password**: `demo`
+
+## Disabled features in demo mode
+The following features are disabled when in demo mode,
+
+- Account registration
+  - Users cannot create new accounts. Everyone uses the shared demo account.
+- Google authentication
+  - The Google sign-in option is removed from the login interface.
+
+## Enabling Demo Mode
+If you want to enable demo mode for your own server, set the following environment variable for the `booklogr-web` service:
+
+```env
+BL_DEMO_MODE=true
+```