Epic - Document the use of ember-page-title

(Issue edited by @jenweber)

Whenever you generate a new route, they have `{{page-title}}` in them! We should update our code examples to include this new boilerplate, and we need to let devs know what to do with it.

There are several places to update. If you want to fix one or many of them, please comment with the tasks you are working on, so we don't have duplicate work. When you open a PR, you should expect the Percy tests to fail - this is because there's a visual difference in the guides due to your writing, and that's a good thing! Maintainers will review the diff and approve or request changes, so you don't need to worry about it.

## Work to do in this repository
- [x] [This accessibility guide page](https://guides.emberjs.com/release/accessibility/page-template-considerations/) shows you how to make your own page title helper. Instead, it should show how to use `{{page-title}}`, and we should delete the sections that show how to make a helper by hand. We should keep the list of addons recommendations, since they have more features than this new helper. `{{page-title}}` comes from [ember-page-title](https://github.com/ember-cli/ember-page-title), which is an addon that is installed by default in new apps. Contributors should only edit the page found in the `release` directory, and not older guides versions.
- [x] Update code examples in the page [Defining your route](https://guides.emberjs.com/release/routing/defining-your-routes/) to include uses of {{page-title}}. Also explain that updating the page title with the current page name will help users with assistive technology know where they are in the website. Contributors should only edit the page found in the `release` directory, and not older guides versions.
- [x] Update the code example in the Quickstart section called [Define a route](https://guides.emberjs.com/release/getting-started/quick-start/#toc_define-a-route) to include page-title. Contributors should only edit the page found in the `release` directory, and not older guides versions.

## Work to do in the welcome-page-addon repository
- [ ] Update [ember-welcome-page](https://github.com/ember-cli/ember-welcome-page) addon and WelcomePage component to explain the contents of the fresh project a bit better. The component should explain the presence of {{page-title}} and link to the "Page Title" section in Page Template Considerations. The welcome page addon provides the content you see when you generate a fresh Ember app and start up your server. Track the work in this ticket: https://github.com/ember-cli/ember-welcome-page/issues/344

## Work to do in ember source code
- [ ] We should add a documentation comment to the ember source code that links to the `ember-page-title` addon. It's very confusing that you can't search the API docs for it. Please track your work in this ticket: https://github.com/emberjs/ember.js/issues/19484 Follow [these instructions](https://github.com/emberjs/ember.js/blob/master/CONTRIBUTING.md#pull-requests) for your commit message and PR.