feat: update .env.sample for clarity

[mwbrooks]

Type of change

• New feature
• Bug fix
• Documentation

Summary

Related to PR slack-samples/bolt-python-assistant-template#44

This pull request updates the .env.sample file to clarify what needs to be set when using the CLI or manually running the app:

• Adds comments above SLACK_APP_TOKEN, SLACK_BOT_TOKEN to clarify that these only need to be set when manually running the app.
• Comments out SLACK_APP_TOKEN, SLACK_BOT_TOKEN by default, since our preference is to use the CLI which auto-sets these values.
• Adds comment above SLACK_API_URL to explain when this should be set.
• Updates OPENAI_API_KEY to clarify that this is required for this template.

Reviewers

# Create the app
$ lack create test-app -t slack-samples/bolt-js-assistant-template -b mwbrooks-env-defaults
$ cd test-app/

# Set environment variables
$ mv .env.sample .env
$ vim .env
# → Set OPENAI_API_KEY

# Test the app
$ lack run
# → Confirm you can roll the dice to hit the OpenAI API

# Clean up
$ lack delete -f
$ cd ..
$ rm -rf test-app

Requirements

• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct

[zimeg]

@mwbrooks LGTM! No comments to make - this is nice and focused to what this sample shows 👾 ✨

[mwbrooks]

Thanks for the quick review and confidence check @zimeg! 🚀