docs(keyring) add recovery key for 3.0

src/gateway/admin-api/db-encryption.md

@@ -112,6 +112,40 @@ HTTP 201 Created
 HTTP 201 Created
 ```
 
+## Recovery Keyring from Database
+
+*This endpoint is only available with the `cluster` keyring strategy.*
+
+*The endpoint requires that the `keyring_recovery_public_key` Kong configuration value is defined.*
+
+**Endpoint**
+
+<div class="endpoint post">/keyring/recover</div>
+
+**Request Body**
+
+| Attribute                 | Description    |
+| ---------                 | -----------    |
+| `recovery_private_key`    | Private key.   |
+
+**Response**
+
+```
+HTTP 200 OK
+```
+
+```json
+{
+    "message": "successfully recovered 1 keys",
+    "recovered": [
+        "RfsDJ2Ol"
+    ],
+    "not_recovered": [
+        "xSD219lH"
+    ]
+}
+```
+
 ## Generate New Key
 
 *This endpoint is only available with the `cluster` keyring strategy.*

src/gateway/plan-and-deploy/security/db-encryption.md

@@ -109,6 +109,29 @@ The exported keyring should be stored in a safe location for disaster recovery
 purposes. It is not designed to be modified or decrypted before being used during
 a disaster recovery process.
 
+### Restore the Keyring
+
+As the Keyring material is encrypted with a randomly-generated symmetric key, and only stored in memory,
+the Keyring could be lost or corrupted if the Kong process is restarted.
+Either way, the Keyring can be restored by importing the exported keyring.
+
+A recommended procedure is to use the Recovery mode, you should set the `keyring_recovery_public_key`
+Kong configuration value to the public key of the recovery keypair.
+(You can also follow the Generate a Management RSA Key Pair guide to generate a keypair for this purpose.)
+
+The Keyring material is encrypted with the public RSA key defined via the `keyring_recovery_public_key`
+Kong configuration value in the database, the recovery keypair then is used to decrypt the Keyring material in the database.
+
+This will restore the Keyring material to the cluster automatically without manual backup.
+
+```bash
+$ curl -X POST localhost:8001/keyring/recover -d "recovery_private_key=$(cat recovery_priv.key)"
+{
+  "id": "500pIquV",
+  "key": "3I23Ben5m7qKcCA/PK7rnsNeD3kI4IPtA6ki7YjAgKA="
+}
+```
+
 ### Exercise the Encryption Routines
 
 Create a Consumer with a basic-auth credential. At this point, the `password` field of the basic-auth credential will be symmetrically encrypted before it is written to the database (in addition to being hashed by the basic-auth plugin, which is done by the plugin regardless of whether keyring encryption is enabled):