docs: use staging openapi.json and align reference page with scraper output

- Replace openapi.json with staging1 version, which includes the new
  GET /2/dm_conversations/media/{dm_id}/{media_id}/{resource_id} endpoint
- Rename get-dm-media.mdx -> download-dm-media.mdx to match the filename
  the @mintlify/scraping workflow generates from operationId
  'dmConversationsMediaDownload', so future automated runs stay in sync
- Update docs.json, introduction.mdx, and migrate.mdx to reference
  the new filename

Author: tcaldwell-x

docs.json

@@ -533,7 +533,7 @@
                           "x-api/direct-messages/media/migrate"
                         ]
                       },
-                      "x-api/direct-messages/get-dm-media"
+                      "x-api/direct-messages/download-dm-media"
                     ]
                   },
                   {