Simplify the profilecli query merge command

_Originally posted by @cyriltovena in https://github.com/grafana/pyroscope/pull/2654#discussion_r1391120449_

Add flags (like `-mem` or `-cpu`) to simplify the usage of the `profilecli query merge` command.

We should also most likely show using go how to open and read those merged profiles. I suspect most people don't know how to use pprof.

Start at the profilecli query merge command and inspect how query flags are currently passed. Define and document simplified flags such as -mem and -cpu, then verify that merged profiles can be opened and read with Go and that the command usage explains the workflow.

Written by the indexing model from the issue text.