Comprehensive README with developer and admin guides

Complete rewrite including:
- DefectDojo access instructions (URL, login, API token)
- Developer guide for adding security scanning to repos
- Understanding and managing scan results
- Available workflows with parameters
- Architecture diagram
- Admin guide (SSH, backups, deployments, secrets)
- Troubleshooting section

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

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: stevemidgley