feat: add responsive library landing page and refresh README banner

[NicooleT]

Summary

Add a standalone, responsive landing page for React Native Safe Area Context in website/, based on the desktop and mobile Figma designs. The site links to the existing documentation and API reference and is configured for a potential rrsac.appandflow.com deployment.

• Add desktop and mobile artwork, bundled fonts, compact mobile navigation, and vertically centered sticky artwork on desktop.
• Add an install-command copy button with animated success feedback, a two-second reset, accessible announcements, and a visible error fallback.
• Add press feedback for Get started and fading text/underline hover states with click targets limited to feature titles and icons.
• Organize the page into reusable components, shared content, and a CSS module, with only state-dependent animation values inline.
• Replace the README banner with the new artwork.

This PR adds the site and its build configuration; it does not deploy the site or change domain/DNS settings.

Test Plan

• yarn --cwd website build — passed.
• Compared desktop and mobile screenshots before and after the component/CSS refactors. Screenshots matched exactly at 1600px, 1100px, and 402px; the 320px layout was unchanged, with minor rasterization differences inside the phone artwork.
• Checked desktop sticky centering and mobile layouts, including horizontal overflow.
• Verified clipboard contents, success animation, automatic reset, stable layout, and clipboard-error fallback.
• Verified Get started press scaling, link hover colors/underlines, and feature arrow removal.
• Checked the documentation and API link destinations.