The markdown renderer passes its output through an HTML pretty printer.
While this is good in most cases, it makes a mess of elements that expect
their content to be left untouched.
The pretty printer already ignores `pre` tags (and other built-ins) by
default. This fix allows us to specify other tags that should be left
alone.
Further it actually specifies this option for `code-example` and `code-pane`
tags, which expect to contain preformatted content.
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