add prefixHeaderId to on-this-page links

[mansona]

In the main ember guides we use showdown config to set a prefix for the header ids so that we didn't break links when we migrated to use Guidemaker for the guides

https://github.com/ember-learn/guides-source/blob/master/config/environment.js#L42-L45

This config needs to be read (if it exists) and set as a prefix to the links in the on-this-page component 👍

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[github-actions[bot]]

Some tests with 'continue-on-error: true' have failed:

• ember-release-no-deprecations

  • ember-beta
  • ember-canary

  Created by continue-on-error-comment