Preserve heading levels for Part source files

[alanlujan91]

Updated the mdast transformation to retain original heading levels for files of kind 'Part', ensuring structural sections like appendices are not altered. Other file kinds continue to adjust heading depth based on title and frontmatter.

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: 75e082fb91a005223fd94df0e50fd2978ec30db6

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.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

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