Fix v2 doc errors and add missing integration guidance

[JordanHood]

Summary

Fixes factual errors in v2 partner docs and adds missing integration guidance that was stripped from the API schema (companion to holidayextras/partner-api PR).

Errors fixed:

• Cancellation quote endpoint: PATCH -> GET (Workflow 4)
• Parameter names: parking_entry_date_time -> parking_entry_datetime, outbound_flight_date_time -> outbound_departure_datetime, etc.
• Stage 5: removed non-existent confirmation-details endpoint, replaced with hosted page / build-your-own guidance
• Stage 1: removed UNLOCODE reference (only IATA supported)
• Stage 6: fixed booking status enum values
• Webhook guide: fixed wrong API endpoint reference
• Auth doc: removed empty heading

Missing guidance added:

• Date and time formats section (local datetime = airport timezone, UTC for tokens/policies)
• Partner reference max length (20 chars)
• amendable_data field explanation
• Refund policies evaluation logic
• confirmation_page_link availability clarified (GET response, not POST)

Index page updated:

• Added production and staging OpenAPI schema URLs
• Better documentation links with descriptions
• Removed internal infrastructure domains

Test plan

• Verify links are not broken
• Review rendered markdown for formatting issues
• Cross-reference endpoint docs against OpenAPI schema

[samuelwisemanhx]

Failing Ruby action, but nice improvement to the docs