docs(preview): add bulk contacts endpoint and merged contact 410 response

Documents two new Preview API features:

1. POST /contacts/bulk and GET /contacts/bulk/{id} — new bulk update endpoint
   with clean request format, async job tracking, and status polling (PR #495002)

2. 410 Gone with Link header for merged contacts on GET /contacts/{id} and
   GET /contacts/find_by_external_id/{external_id} — returns RFC 8288 Link
   header pointing to canonical contact (PR #497262)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kstenson