Confusion around setting up Git Gateway + Identity for use in Decap CMS

_related https://github.com/netlify/netlify-cms/issues/3711_

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I am in the process of installing **NetlifyCMS + Netlify Large Media** and wanted to log a bit of confusion I am encountering in the docs around the `git-gateway` setup.

## My path
1. I started on the NetlifyCMS page for setting up [Netlify Large Media](https://www.netlifycms.org/docs/netlify-large-media/)
1. On the second requirement, I navigated to [Configure Netlify CMS to use the Git Gateway backend with Netlify Identity](https://www.netlifycms.org/docs/git-gateway-backend/#git-gateway-with-netlify)
1. From the Git Gateway page, I then went to the [Netlify Identity Docs](https://docs.netlify.com/visitor-access/identity/#enable-identity-in-the-ui)
1. Then I noticed that it mentioned using the `netlify-identity-widget` to integrate the into your site, but was not sure if it was already included in the NetlifyCMS JS and set up, since this was not mentioned in any of the previous docs for setting up NetlifyCMS + Large Media + Git Gateway.
2. Also, in looking at the code samples on the [Netlify Identity Widget README](https://github.com/netlify/netlify-identity-widget), it looked like this was the code we'd use to should need to put a login button on the _public facing_ side of our site. The readme didn't mention NetlifyCMS or indicate if this would go in the `/admin/index.html` page for NetlifyCMS. So I skipped this step, knowing that I could come back to it later.
3. Next, I changed the backend setting in our config to use `name: git-gateway` and enabled "Identity" in the Netlify admin. Then I
- I went to our page at `https://brooklynrail.netlify.app/workflow`
- "Signed up" for an account
- received an email with a link to authenticate
- clicked the link to authenticate
- verified that my account was created in the Netlify Identity Admin ✅
- tried to log in using the new account I set up `https://brooklynrail.netlify.app/workflow` and got "Email not confirmed" 🛑

### Wrong turn, going back
So now I am going back to installing the [Netlify Identity Widget](https://github.com/netlify/netlify-identity-widget) in our CMS admin page, and have a few questions about the code on the readme:
1. [Of the code that is here](https://github.com/netlify/netlify-identity-widget), what is the minimum that I need to install on my page for NetlifyCMS?
1. The example of the `global object`, is that needed in my site, or is that just an example?
1. Do I need to configure "roles" to get this working?

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## Recommendations for adding Clarity in the docs
I am taking some notes locally and am happy to submit a PR when this is all done. Here are some initial thoughts on improvements

1. Include all of the steps for configuring up Netlify Large Media on the [main docs page](https://www.netlifycms.org/docs/netlify-large-media/),
1. include the steps to set up Git Gateway and Identity and Identity Widget
1. include each of the command line installs that are needed as part of the steps _(e.g. `netlify-cli`)_
1. Include screenshots at each step to make it clear you're in the right place
1. Include a link to an example repo with the configuration already set up

❤️🌈