docs(markdown-it): work around fence rule pre code wrapper, close #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.

[netlify]

✅ Deploy Preview for shiki-matsu ready!

Name	Link
🔨 Latest commit	1039826
🔍 Latest deploy log	https://app.netlify.com/sites/shiki-matsu/deploys/68071fc2487ae400086b29fa
😎 Deploy Preview	https://deploy-preview-955--shiki-matsu.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[netlify]

✅ Deploy Preview for shiki-next ready!

Name	Link
🔨 Latest commit	1039826
🔍 Latest deploy log	https://app.netlify.com/sites/shiki-next/deploys/68071fc2ff5b750008b2b069
😎 Deploy Preview	https://deploy-preview-955--shiki-next.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.