add deploy hook example to heroku guide

[kkemple]

Summary

Adds an example of using the deploy hook to automate deployments to the Heroku guide.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[WilliamBergamin]

I wonder if it would be worth while to move this to a sample and set up a job that actually deploys they app? 🤔
It might make it easier to maintain and tests

[zimeg]

📚 The CLI docs now have reference for "hooks" where we might include similar example soon!

🔗 https://docs.slack.dev/tools/slack-cli/reference/hooks/#deploy