docs(caddy): new page Cluster storage for Caddy

website/docs/3-concepts/2-ingress/4-cluster-storage-for-caddy.md

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+# Cluster storage for Caddy
+
+The [Uncloud storage module](https://github.com/unlabs-dev/caddy-uncloud) lets Caddy instances share TLS certificates,
+private keys, and ACME challenge tokens through Uncloud's cluster store. It uses distributed locks to coordinate
+certificate issuance. Once one instance obtains a certificate, the others can use it too.
+
+## Why use cluster storage?
+
+By default, each Caddy instance stores its certificates locally. When DNS points to multiple machines or a load balancer
+distributes traffic between them, an ACME challenge can reach a different instance from the one requesting the
+certificate. That instance may not have the challenge token, which can delay or prevent certificate issuance.
+
+Cluster storage is optional. The default Caddy image does not include the module, so you need to deploy an image that
+includes it and configure Caddy to use it.
+
+## Enabling cluster storage
+
+:::info Requirements
+
+Cluster storage requires Uncloud **v0.21.0 or newer** for both the `uc` CLI and the daemon on every cluster machine.
+Check your CLI version with `uc version` and daemon versions with `uc machine ls`. Upgrade older versions before
+enabling cluster storage.
+
+:::
+
+Create a global Caddyfile, or add `storage uncloud` to the global options in your existing one:
+
+```caddyfile title="global.Caddyfile"
+{
+    storage uncloud
+
+    # Uncomment to enable debug logs useful for troubleshooting storage operations:
+    # debug
+}
+```
+
+Deploy Caddy with the pre-built module image and your global config:
+
+```shell
+uc caddy deploy --image ghcr.io/unlabs-dev/caddy-uncloud:0.1.1 --caddyfile global.Caddyfile
+```
+
+See the [module README](https://github.com/unlabs-dev/caddy-uncloud#usage) for custom image builds, Compose deployment,
+and additional storage options.
+
+## Verifying storage
+
+Check that `storage uncloud` appears in the Caddy config:
+
+```shell
+uc caddy config
+```
+
+List issued certificates in cluster storage with [`uc caddy cert ls`](../../9-cli-reference/uc_caddy_cert_ls.md):
+
+```shell
+uc caddy cert ls
+```