[DOCS] Improve heroku docs to describe how to make and deploy code/config changes

[marks]

Describe the bug

Add information to Heroku deploy docs on how to modify config.js.

Basically, you'll need to follow https://help.heroku.com/XOBUHLKQ/why-do-i-see-a-message-you-appear-to-have-cloned-an-empty-repository-when-using-heroku-git-clone. Or more specifically:

heroku git:clone -a YOUR_HEROKU_APP_NAME
cd YOUR_HEROKU_APP_NAME
git remote add origin https://github.com/slackapi/template-triage-bot
git pull origin main
# make changes to config.js
git add . && git commit -m "a useful commit message"
git push

Requirements (place an x in each of the [ ])**

• 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.
• I've searched for any related issues and avoided creating a duplicate issue.