docs(router): explain Webpack SPA fallback configuration

[LadyBluenotes]

Goal

Document the Webpack settings required for client-side routing to work on direct loads and refreshes.

Scope

• Explain devServer.historyApiFallback for serving the application shell on nested routes.
• Explain output.publicPath, including how deployments under a subpath need their actual public base.
• Keep the maintained React and Solid Webpack quickstarts aligned with the guide.
• Add Webpack E2E coverage for a direct load or refresh at a nested route.

Related Work

This continues the documentation work from #4677