Revise maintaining-and-tuning/configuration/file-organization.md

The doc page should differentiate between default paths for client and server installations.

Default path for server installations is indicated to be `/etc/grr/grr_server.yaml`, which is incorrect. It is actually `/usr/share/grr-server/install_data/etc/grr-server.yaml`.

`Client.control_urls` is mentioned in the doc page; This config option is deprecated and should be deleted (from the GRR codebase, as well as the documentation).