docs: add getting started, LocalStack/Floci setup, architecture, and contributing guides

[JSisques]

Closes #109
Closes #110
Closes #111
Closes #112

Type

• Documentation only

Summary

• Add docs/getting-started.md — prerequisites, install, env config, dev server, tests (docs: getting started guide #109)
• Add docs/localstack-setup.md — covers both Floci (recommended, free MIT) and LocalStack as emulator options, endpoint precedence, troubleshooting (docs: LocalStack setup guide #110)
• Add docs/architecture.md — feature-sliced layout, Mermaid diagrams, AWS SDK server-side boundary, i18n routing (docs: architecture overview #111)
• Add docs/contributing.md — branching, conventional commits, TDD, responsive UI requirements, link to AGENTS.md (docs: contributing guide #112)
• Add root CONTRIBUTING.md stub (GitHub auto-surfaces on PRs)
• Update README.md: Documentation section + pnpm-only dev command

Changes

File	Change
docs/getting-started.md	New — issue #109
docs/localstack-setup.md	New — issue #110, covers Floci + LocalStack
docs/architecture.md	New — issue #111, Mermaid diagrams
docs/contributing.md	New — issue #112
CONTRIBUTING.md	New root stub
README.md	Add Documentation section, pnpm-only dev command

Test Plan

• All relative links verified to resolve to existing files
• Mermaid syntax validated (two flowchart blocks)
• No existing README content removed
• Conventional commit format
• No Co-Authored-By trailers