Add explanatory boxes

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From https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Using_HTML_sections_and_outlines:

The HTML Aside Section Element (<aside>) defines a section that, though related to the main element, doesn't belong to the main flow, like an explanation box or an advertisement. It has its own outline, but doesn't belong to the main one.

[riccardoscalco]

Again on <aside>: see example in https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-small-element. Aside is used for advertisement block, content that is multiple paragraphs long and is removed from the main flow, aside goes between sections