docs(start): explain parent route outlets

[coyaSONG]

Summary

• explain that generated parent route components must render an Outlet
• add matching parent-route examples for React and Solid Start
• document the automatic-outlet alternative when no shared layout is needed

Why

The route generator creates new files with a starter component. When a file such
as posts.tsx becomes the parent of posts/index.tsx and
posts/$postId.tsx, that component replaces the router's automatic outlet. The
child route still matches, but its content cannot appear unless the parent
component renders <Outlet />.

This puts the fix next to the route-file table where users first create that
hierarchy, and also explains that removing the generated component option
restores the automatic outlet.

Closes #5351

Validation

• pnpm exec prettier --experimental-cli --check docs/start/framework/react/guide/routing.md docs/start/framework/solid/guide/routing.md
• pnpm test:docs (244 Markdown files, no broken links)
• CI=1 NX_DAEMON=false pnpm nx affected --target=test:eslint --exclude='examples/**,e2e/**' --outputStyle=stream --skipRemoteCache (no affected package tasks for the docs-only diff)
• CI=1 NX_DAEMON=false pnpm nx affected --target=test:types --exclude='examples/**' --outputStyle=stream --skipRemoteCache (no affected package tasks for the docs-only diff)
• CI=1 NX_DAEMON=false pnpm nx affected --target=test:unit --exclude='examples/**,e2e/**' --outputStyle=stream --skipRemoteCache (no affected package tasks for the docs-only diff)

Disclosure

This contribution was prepared with OpenAI Codex assistance and manually
verified against the route-generator implementation and the repository's
documentation guidance.

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