docs: migrate docs site to rspress

[adamTrz]

Summary

Migrates the docs site from Docusaurus to Rspress using Callstack's Rspress theme/preset.

Changes

• moved 5.x and 6.x docs to Rspress multi-version setup
• kept legacy 1.x-4.x docs available from the version selector
• ported Docusaurus-specific components and docs helpers to Rspress-compatible equivalents
• added Paper-specific homepage, showcase, version selector, and 6.x prerelease notice
• preserved static assets and GitHub Pages base-path behavior

Validation

• yarn --cwd docs build
• local manual review of homepage, version navigation, component docs, showcase, and prerelease banner

Screen.Recording.2026-06-17.at.09.19.30.mov

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