docs: add parser integration guide and link in README ...

- Create docs/PARSER_INTEGRATION.md with:
  - Endpoint selection guide (/ingest vs /with-images)
  - Full Python integration example with VmkCollectorClient
  - Response handling (completed, invalid, failed)
  - Retry logic and rate limits
  - FAQ for parser developers
- Add link to README in API for parsers section

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