style: update "on this page" to only show headers up until level 3 and indent

[IgnaceMaes]

Implements https://github.com/ember-learn/guidemaker-ember-template/issues/179

• Only shows h1, h2, and h3 in the "on this page" section
• Indents h3 levels with left padding
  • h2 not indented as I believe it should be the base and h1 should be kept for the title only (see related fix: https://github.com/ember-learn/guides-source/pull/2012)

Before	After

To be fair, this example doesn't properly showcase the real difference, and a real content page would be better suited to test.

EDIT: test using the Template Tag Format content:

Before	After

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