Add sample usage for accessing secret from Secret Manager

Moved from [here](https://github.com/googleapis/google-cloud-node/issues/6530).

### Please make sure you have searched for information in the following guides.

- [x] Search the issues already opened: https://github.com/GoogleCloudPlatform/google-cloud-node/issues
- [x] Check our Troubleshooting guide: https://googlecloudplatform.github.io/google-cloud-node/#/docs/guides/troubleshooting
- [x] Check our FAQ: https://googlecloudplatform.github.io/google-cloud-node/#/docs/guides/faq
- [x] Check our libraries HOW-TO: https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md
- [x] Check out our authentication guide: https://github.com/googleapis/google-auth-library-nodejs
- [x] Check out handwritten samples for many of our APIs: https://github.com/GoogleCloudPlatform/nodejs-docs-samples

### Documentation Request

The [Secret Manager NPM page](https://www.npmjs.com/package/@google-cloud/secret-manager) could use a simple example of the commonly used method for retrieving values from Secret Manager.