Move the ticket-voucher API into its own standalone doc
(docs/fluentforms-ticket-vouchers.md) — endpoint, 0/1/2 rules, config, form
snippet, and curl test — so it's easy to find and hand off for the lookup. The
donor-discount doc now links to it instead of duplicating the section.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
