docs: add contribution guidelines and developer setup docs

[Andreschuks101]

Description

The README referenced a CONTRIBUTING.md that did not exist. This PR adds comprehensive
contributor and developer documentation covering setup, standards, the PR process, testing,
commit conventions, and an architecture overview.

Closes #78

What's included

• CONTRIBUTING.md — code of conduct, ways to contribute, project layout, getting
  started, development workflow, coding standards (TypeScript + Rust/Soroban), Conventional
  Commits, testing requirements, PR process, and review process.
• docs/ARCHITECTURE.md — system diagram and descriptions of the contracts, backend, and
  frontend packages, data stores/external services, request lifecycle, and CI overview.
• docs/DEVELOPMENT.md — prerequisites and step-by-step local setup for all three
  packages (contracts, backend, frontend), env configuration, supporting services, and
  troubleshooting.
• docs/TESTING.md — unit, integration, and E2E conventions for Jest (backend/frontend)
  and Cargo (contracts), with examples and CI guidance.
• .github/PULL_REQUEST_TEMPLATE.md — PR template with a checklist for tests, docs, and
  breaking changes.
• README.md — new Documentation section linking all of the above.

All content is derived from the actual repository (pnpm workspaces, each package's real
scripts, the CI pipeline, and the Soroban/Rust contracts) and is documentation-only — no
source code or build behavior is changed.

Type of change

• Documentation update

Checklist

• All docs are linked from the README
• CONTRIBUTING.md covers code of conduct, setup, PR flow, and review process
• ARCHITECTURE.md includes a system diagram and component descriptions
• DEVELOPMENT.md covers local setup for all 3 packages
• TESTING.md covers unit, integration, and E2E conventions
• PR template includes a checklist (tests, docs, breaking changes)