Docs: improve HTML preview in editor

## Feature Request

Using an editor to display HTML is heavy.

## TODO

1. Investigate: https://github.com/ajaxorg/ace/blob/master/demo/static-highlighter/server.js

## Refactor
1. Just use a `div` with HTML
2. Print with `prettier`
3. Highlight it

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Extracted from @levithomason comment in #473.