Document bulk create and bulk delete contacts (Preview) (#577)

* Document bulk create and bulk delete contacts (Preview)

Adds POST /contacts/bulk (strict create) and POST /contacts/bulk/delete
(permanent delete), mirroring the existing bulk update endpoint.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Align bulk contact create/update fields with the single contact endpoints

Adds avatar, owner_id, unsubscribed_from_emails, and tags (add/remove) to
bulk create/update request bodies to match the single contact create/update
endpoints. Renames the update identifier field to external_id and owner_id
to string to match the rest of the contact schemas, drops the undocumented
companies/company properties, and clarifies tag id requirements and
best-effort application behavior.

* Match Preview bulk contacts spec to developer-docs (source of truth)

Align the bulk create/update/delete contact sections with developer-docs
PR #1018: trim endpoint descriptions, drop the phone example and reorder
bulk_update fields, and move /contacts/bulk/delete ahead of
/contacts/bulk/{id}. Sections are now byte-identical to the docs source of truth.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: VarshSuresh