docs(start): add styled-components SSR example

[LadyBluenotes]

Goal

Add a maintained React Start example for styled-components with server-rendered styles.

Scope

• Base the example on the current React Start example structure.
• Declare the styled-components runtime and build-time dependencies.
• Use the current Start server entry and request lifecycle.
• Keep stylesheet collection request-scoped and release resources after rendering.
• Add an E2E test that verifies styles are present in the initial HTML and remain correct after hydration.
• Keep the example focused on styled-components SSR.

Related Work

This continues the work started in #4394