Document local verification steps for frontend, backend, and UI changes

[forthfate]

Summary

The repository documents individual commands, but a first-time contributor does not have a quick guide to which checks to run for a documentation, frontend, backend, or UI end-to-end change.

Goal

Add a concise local-verification section to the contribution guide.

Scope

• Describe the minimum relevant checks for documentation-only, frontend, and backend changes.
• Include the existing lint, frontend build, Ruff, and pytest commands.
• Briefly explain that the Playwright UI specs need a locally running app and the expected sample data; do not imply they run automatically in CI.
• Keep the section short and link to existing instructions rather than duplicating them.

Acceptance criteria

• A newcomer can choose the appropriate checks for a small contribution.
• Commands match the scripts and configuration currently in this repository.
• The guide clearly distinguishes regular automated checks from the locally prepared Playwright UI checks.
• No application behavior or CI configuration changes are required.

Helpful files

• CONTRIBUTING.md
• README.md
• frontend/e2e/orbit-ui.spec.ts
• .github/workflows/ci.yml