Add comprehensive documentation

- Add docs/api-reference.md with complete API documentation for all
  functions including colony, executor, process, workflow, channel,
  blueprint management, and core types
- Add docs/tutorial.md with getting started guide, examples for
  submitting processes, building executors, workflows, and channels
- Update README.md with improved structure, API overview section,
  and links to new documentation

Author: johankristianss