[#220] Add deployment guide for Heroku

[mwbrooks]

Summary

This pull request is part of the on-going work related to #220. After reviewing, we will want to squash this merge.

This pull request adds the following:

• Updates side bar with a "Deploying" section
• Adds a "Deploying to Heroku" guide
• Adds a "Getting Started with Heroku" example app to bolt-js/examples/

Remaining tasks before merging:

• Walk through tutorial from beginning to end to ensure steps are accurate (@mwbrooks)
• Team discussion on how and where to host example apps
  • Update mwbrooks/bolt-js-getting-started-app
  • Update mwbrooks/bolt-js-getting-started-with-heroku-app
• Team review of outstanding questions (see GitHub comments)

After merging, we will want to:

• Translate the side bar for the Japanese documentation
• Translate the guide for the Japanese documentation

Preview

→ https://mwbrooks.github.io/bolt-js/deployments/heroku

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.

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[mwbrooks]

@shaydewael I added two images to help explain where to update the Request URL in the app management page. I know you're thinking about improving the images in the Getting Started guide. I don't want these images to step on the toes of your work, so let me know if there's a better approach that you have in mind.

[seratch]

This is already a great guide 👍

[shaydewael]

We should make this section header a conditional until there's a translated version

[mwbrooks]

Added commit cef89ab to only display the Deployments section when there is content (including hiding the section header). Currently, this will hide the Deployments section for the Japanese translate.

@shaydewael my Jekyll/Liquid skills 🔪 are a little rusty. Would you mind looking at my implementation? If you have a more elegant way to do it, I'd love to see it!

[misscoded]

This is great and so important. It's going to help provide such a seamless experience for people that want to use Heroku to host. 🎉

[seratch]

Regarding the place to have the example app, as I mentioned in a comment, my suggestion is to have it under almost unused examples dir in this repo. But other options are also great. Apart from that, all look great to me.

[seratch]

approved again - great work!