[V3 REST API] Removed GET /document-parts/ endpoints

This change removes two Beta v3 API endpoints for fetching Document Parts:

• GET /document-parts/{id}
• GET /document-parts/items

The same Document Part data can instead be returned by querying the following Entry-based endpoint instead:

• GET /entry/{entry_id}/parts/items

To get a single Document Part, use the parameter id.anyOf to filter results to individual Document Part ids.