Core: Add new tags to distinguish docs attachment

[tmeasday]

Part of fix for #21798

What I did

• Add two tags attached-mdx and unattached-mdx so we can tell if a MDX file it attached without rendering it
• Ensure the attached CSF file is always first so we know what it is.

Now all docs files have the docs tag and one of:

• stories-mdx
• autodocs
• attached-mdx
• unattached-mdx.

How to test

See tests, check sandbox indexes.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]