docs: correct custom server optimization guidance

[timneutkens]

Summary

• remove the inaccurate claim that a custom server disables Automatic Static Optimization
• retain the guidance to use a custom server only when the integrated router cannot meet an app's requirements

Verification

• pnpm prettier --with-node-modules --ignore-path .prettierignore --check docs/01-app/02-guides/custom-server.mdx
• commit hook: prettier --write and eslint --fix for the changed MDX file

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