feat: maintenance mode docs (#3159)

feat: maintenance mode docs

website/docs/reference/maintenance-mode.mdx

@@ -0,0 +1,27 @@
+---
+title: Maintenance Mode
+---
+
+:::info Availability
+
+Maintenance mode was introduced in Unleash 4.21.0.
+
+:::
+
+Unleash maintenance mode is a feature that lets administrators put Unleash into a mostly read-only mode. While Unleash is in maintenance mode: 
+- Unleash users **can not change any configuration settings**
+- Unleash's APIs will not allow you to persist any changes
+
+However, any metrics sent to Unleash from client SDKs are still processed, so this mode does not have any effect on client SDKs.
+
+Maintenance mode is particularly useful during important events when any accidental or deliberate changes the feature/strategy configurations could lead to issues.
+
+## Maintenance mode configuration
+
+Maintenance mode is controlled from the "maintenance" section of the Unleash admin configuration page.
+
+![Maintenance mode being toggled on](/img/maintenance-config.png)
+
+When maintenance mode is enabled, a warning banner appears at the top of the Unleash dashboard, indicating that any changes made during this period will not be saved and may result in errors.
+
+![Maintenance mode banner when maintenance mod is toggled on](/img/maintenance-banner.png)

website/sidebars.js

@@ -329,6 +329,7 @@ module.exports = {
                         'reference/technical-debt',
                         'reference/unleash-context',
                         'reference/change-requests',
+                        'reference/maintenance-mode',
                     ],
                 },
                 'generated/unleash-edge',