Add DropZone docs to admin extensions

[sam-b-rose]

Background

Part of https://github.com/shop/issues-polaris/issues/1229

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset directory. If the changes are user-facing and should cause a version bump, run yarn changeset to track your changes and include them in the next release CHANGELOG. If you are making simple updates to repo configuration, examples, or documentation, you do not need to add a changeset.