feat: document cli create-key command

Author: florian-lefebvre

src/content/docs/en/reference/cli-reference.mdx

@@ -376,6 +376,10 @@ astro telemetry reset
 Add the `astro telemetry disable` command to your CI scripts or set the `ASTRO_TELEMETRY_DISABLED` environment variable.
 :::
 
+## `astro create-key`
+
+Generates a key to encrypt props passed to Server islands, see [how to reuse the encryption key](/en/guides/server-islands/#reusing-the-encryption-key).
+
 ## Common flags
 
 ### `--root <path>`