Add comprehensive draft and preview access documentation

Extends the documentation with detailed examples for enabling draft access:

## New Content
- Complete "Draft & Preview Access Guide" covering:
  - Basic authenticated draft access
  - Author-only draft viewing
  - Headless WordPress preview modes
  - Time-limited preview tokens
  - Next.js preview mode integration
  - Scheduled posts access
  - Private posts handling

## Enhanced Filter Documentation
Added 7 new examples to vip_block_data_api__rest_validate_post_id:
- Allow authenticated users to view drafts
- Allow post authors to view their own drafts
- Allow editors to preview with specific capabilities
- Allow draft access with time-limited preview tokens
- Role-based draft access
- Content preview for headless CMS
- Scheduled posts (future) access

## Security Best Practices
- Token validation with constant-time comparison
- Permission checking examples
- Rate limiting implementation
- Input sanitization
- Time-limited token generation

This addresses common use cases for headless WordPress applications
that need to preview draft content before publishing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: whyisjake