Add sample usage for accessing secret from Secret Manager

[danieljbruce]

Moved from here.

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 could use a simple example of the commonly used method for retrieving values from Secret Manager.