Improve auto-generated Netlify redirect file

Recently we got an email from netlify saying that they are changing the behaviour of the redirects.

Currently, if you have a redirect defined as follows:

```
/v3.16.0/object-model/computed-properties /release/components/component-state-and-actions
```

and you also have an index file in the location:

```
/v3.16.0/object-model/computed-properties/index.html
```

Netlify will ignore the file, and will respond with a 301 redirect to `/release/components/component-state-and-actions`. Apparently this behaviour was a bug and the intended behaviour was to ship the file if it exists. More information can be read [in their blog post](https://community.netlify.com/t/changed-behavior-in-redirects/10084) about the issue.

Myself and @locks have checked our build output from Guidemaker and it looks like we **do** generate files in these locations so we will be affected. On further inspection, however, it looks like the generated files have the correct redirect information in them:

```html

```

This means that a user will not notice any difference after Netlify changes the default behaviour.

We think however that it's probably best to make sure that we only have the netlify redirect and not the generated `index.html` file in our current setup. It's not clear if this is something that we should be able to configure in [fastboot](https://github.com/ember-fastboot/fastboot), [prember](https://github.com/ef4/prember), or if this is something that [Guidemaker](https://github.com/empress/guidemaker) should be able to fix.