Add contributor guidelines (CONTRIBUTING.md + PR template)

[dfederm]

Summary

Adds human-facing contribution guidance for JellyBox so future PRs are easier to scope, validate, and review. This is a docs-only change: it adds CONTRIBUTING.md and .github/PULL_REQUEST_TEMPLATE.md with no source changes and no build impact.

What's included

• Guidance to keep PRs single-purpose and reviewable.
• The expected MSBuild validation gate: x64 build, restore-on-build, and 0 warnings/0 errors as the bar because there are no automated tests.
• Manual testing expectations for UI, playback, and controller changes, including the platform tested.
• PR description expectations, including screenshots or recordings for UI changes.
• Key project conventions documented inline for human contributors, including UWP-not-WinUI3 and central package management.

Validation

Docs-only change, so no build or test gate was run. Concrete repo claims were checked against the existing project configuration and guidance, including .github/copilot-instructions.md, .github/workflows/build.yml, Directory.Build.rsp, Directory.Packages.props, and JellyBox.sln.