docs(desktop): document React Router support

[akazwz]

Summary

• Add React Router to the desktop and compile framework support docs
• Document the Deno-compatible app/entry.server.tsx that uses renderToReadableStream
• Clarify that React Router SPA and SSR projects must build successfully under Deno before packaging

This is the docs half of the React Router desktop support work and should land together with the Deno CLI/runtime support change.

Runtime PR: denoland/deno#35557

Testing

• deno fmt --check runtime/desktop/frameworks.md runtime/desktop/index.md runtime/reference/cli/compile.md runtime/reference/cli/desktop.md
• deno task check:freshness
• deno test -A frontmatter_test.ts
• deno task test