refactor: align repository with Go project layout

[skyoo2003]

Summary

Aligns the repository with the community Standard Go Project Layout and records the layout rules in the contributor guide.

Related Issue

N/A

Changes

• Move the code generator, Smithy models, external compatibility tests, container packaging files, and Compose deployment definition to their standard locations.
• Update build tooling, CI/CD, release configuration, scripts, tests, and documentation to use the new paths.
• Add project-layout placement guidance to CONTRIBUTING.md.

Test Plan

• CGO_ENABLED=0 go test ./... -count=1
• make test-compat (1,527 passed, 2 skipped, 1 xfailed)
• make build
• make codegen (no generated-output drift)
• make docs-build
• docker-compose -f deployments/docker-compose.yml config
• golangci-lint run --timeout=5m

Checklist

• Self-reviewed the code
• Added/updated tests
• Lint/format passes (golangci-lint run)
• Updated documentation (if applicable)
• Added a Changie changelog fragment for user-facing changes (changie new, one sentence and two at most, see RELEASE.md) — or N/A (docs/tests/chore only): N/A (chore)