Update contribution guide

Current contribution guide is here: https://github.com/microsoft/react-native-gallery/wiki/Contributing-to-React-Native-Gallery
Mostly it covers pretty broad contribution topics, not the quirks specific to this repo. That's a fine doc to have... but we really need to capture what you need to know if you're going to work on this app.

Things like:

• Running yarn test:update
• Clearer callout of timeZoneOffsetInSeconds problem (it's a bit buried as is): https://github.com/microsoft/react-native-gallery/issues/441
• Linting expectations (yarn lint clean)
• Accessibility checking with Accessibility Insights 🧪
• Anything else you might hit in the pipelines
• Hotspots for manual testing
• Platform-specific code
• How to add a new sample page (note: covered in the readme)
• App architecture overall

Start with the existing wiki contribution guide and the repository README, then review the listed workflows: yarn test:update, yarn lint, Accessibility Insights, manual testing hotspots, platform-specific code, and app architecture. Check issue #441 for the timeZoneOffsetInSeconds guidance. Done means the guide documents the repo-specific setup, checks, testing areas, sample-page guidance, and architecture without duplicating the README.