Add API usage examples and documentation references

[ivanauth]

Summary

Add comprehensive examples demonstrating common authzed-go operations and link to external documentation resources in the README.

Fixes #142

Changes

• Add examples/example_test.go with 12 runnable examples covering:
  • Connecting to Authzed and SpiceDB
  • Writing schemas and relationships
  • Checking permissions (single and bulk)
  • Looking up resources and subjects
  • Reading and deleting relationships
  • Using caveats (conditional permissions)
  • Watching for changes
• Update README with examples table and external documentation links

Testing

# Build examples
go build ./examples/...

# Run examples (requires local SpiceDB)
docker run --rm -p 50051:50051 authzed/spicedb serve --grpc-preshared-key "somerandomkeyhere"
go test -v ./examples/...