[serve][docs] Document lightweight config updates (#27706)
[serve][docs] Document lightweight config updates (#27706)

A new feature was recently added, where Serve replicas are not restarted if only `num_replicas`, `autoscaling_config`, and/or `user_config` is updated in the config file that's redeployed. Updating docs to talk about this feature.

Co-authored-by: shrekris-anyscale <9361f90eb5dc479bcfd2c31c0711924177b7bf9c@users.noreply.github.com>
ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray