Maintenance mode doc (#1686)

Co-authored-by: Jeff Escalante <[email protected]>
Co-authored-by: victoria <[email protected]>

docs/maintenance-mode.mdx

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+---
+title: Maintenance Mode
+description: Learn about Clerk's Maintenance Mode.
+---
+
+**Maintenance Mode** is a special operational state designed to ensure minimal disruption to signed-in users during critical database upgrades or outages.
+
+## Production instances
+
+Mutation methods (`POST`, `PATCH`, `PUT`, `DELETE`) will be rejected with a `503 Service Unavailable` status, along with an error message informing users that the system is temporarily unavailable. This includes all sign-up and sign-in attempts. Clerk's prebuilt components and the hosted Account Portal will display errors for these requests.
+
+GET requests, as well as session refresh requests ([`/touch`](/docs/reference/frontend-api/tag/Sessions#operation/touchSession) and [`/tokens`](/docs/reference/frontend-api/tag/Sessions#operation/createSessionToken) endpoints), aren't affected. Users with active sessions (already signed in) will not be signed out, and will continue to have access to your app. However, any mutations to their user or org data will return the same `503` error.
+
+## Development instances
+
+During this period, development instances will experience full downtime. Users will be unable to sign up or in, and already signed-in users will be signed out.