Make helptext for adding engines clearer for nontechnical users

[Chewt]

The helptext for adding a new engine had room for improvement to allow for nontechnical users to better understand how to add an engine to Sabaki.

This was brought up in #756 when the author had trouble adding an engine to Sabaki. This PR hopes to clarify confusion from users who may just be trying to follow command line instructions from Engine documentation and not know much about command lines.

[sente361]

Thanks @Chewt. Many people would find this useful. It would also be very useful to include a guide to choosing the best settings in the gtp line and the time settings line. I use the settings below but I have no idea whether or not they are best for my system.
gtp -override-config ponderingEnabled=false,maxVisits=200000 -model KataGo20b-1.1.gz
time_settings 0 10 1;

[Chewt]

The only problem with including a guide on the settings, is that they are specific to the engine itself. So those settings you mentioned are specific to katago. We would either need to include a guide that lists settings for all of the common engines, or link to the documentation to the engines themselves. I would lean toward the second option since that documentation already exists.

[Chewt]

Actually, I think this pretty much covers the basic usecase to get people started: https://github.com/SabakiHQ/Sabaki/blob/master/docs/guides/engines.md

Maybe this could somehow be integrated into the GUI somewhere under the help menu, but that's not really related to this PR