[Documentation] Update install docs to acknowledge systemd
Some of the install docs recommend setting up runit supervision for packages that come with systemd unit files. On systems running systemd, this results in a runit service and a systemd service that fight over the listening port.The arvados-ws and keep-balance install pages are better: they say "if you're running systemd, enable the service; otherwise, set up runit." Other pages should be updated with the same approach (or something better), including
The arvados-git-httpd install docs only provide guidance for systemd, and leave the runit configuration up to the installer. It might be worth adding runit instructions here for consistency.