docs: add section for secrets plugins in legacy_plugins.md

Signed-off-by: Sai Sanjay <saisanjay7660@gmail.com>

Author: sanjay7178

docs/extend/legacy_plugins.md

@@ -80,6 +80,12 @@ The sections below provide an overview of available third-party plugins.
 | [HBM plugin](https://github.com/kassisol/hbm)                        | An authorization plugin that prevents from executing commands with certains parameters.                                                                                                                                                                                                                                              |
 | [Twistlock AuthZ Broker](https://github.com/twistlock/authz)         | A basic extendable authorization plugin that runs directly on the host or inside a container. This plugin allows you to define user policies that it evaluates during authorization. Basic authorization is provided if Docker daemon is started with the --tlsverify flag (username is extracted from the certificate common name). |
 
+### Secrets plugins
+
+| Plugin                                                                           | Description                                                                                                                                       |
+|:---------------------------------------------------------------------------------|:--------------------------------------------------------------------------------------------------------------------------------------------------|
+| [Swarm External Secrets](https://github.com/sugar-org/swarm-external-secrets) | A Docker Swarm secrets plugin that integrates with multiple secret management providers including HashiCorp Vault, AWS Secrets Manager, and more. |
+
 ## Troubleshooting a plugin
 
 If you are having problems with Docker after loading a plugin, ask the authors