[core] Document/improve how to pass system configs

[stephanie-wang]

Description

Some system configs like object spilling directory are quite difficult to override, and we are also not consistent about documenting the system configs.

For example, you may have to specify a system config like this to override object spilling directory:

system-config: '"{\"object_spilling_config\":\"{\\\"type\\\":\\\"filesystem\\\",\\\"params\\\":{\\\"directory_path\\\":\\\"/tmp/spill\\\"}}\"}"'

Ideally we should automatically generate docs from the config def file and have an easier way to pass JSON-like configs.

Use case

No response

[dizhouwu]

def would be useful to users!