docs: correct custom server optimization guidance (#94229)
## 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
