update markdown config docs

[sarah11918]

Changes

This is a corresponding docs change for withastro/docs#9888 that moves some syntax highlighting and Shiki config documentation into the configuration reference docs (instead of just linking to the guide)

Testing

no tests; docs

Docs

only changes current configuration reference docs generated

[changeset-bot]

⚠️ No Changeset found

Latest commit: 3511d15

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[codspeed-hq]

CodSpeed Performance Report

Merging #12382 will not alter performance

_{Comparing sarah11918-patch-1 (3511d15) with main (e10b03e)}

Summary

✅ 1 untouched benchmarks