docs(markdown-it): work around fence rule pre code wrapper [#811]

[olets]

Description

markdown-it wraps code block html in <pre><code>.  That can be undesirable when using a Shiki transformer which adds its own <pre><code>. This PR

• documents this fact
• points to the markdown-it discussion which is the closest thing to official documentation of how to work around this
• and links to the package I put together based on that following discussion in a Shiki issue

Linked Issues

• Closes using a transformer to add a wrapping element results in extra duplicative nodes #811

Additional context

If you decide to close without merging, please also close #811.

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