clarify what "30 minutes of non-interactive use before project stops" means (was: Projects are shutting down quickly (site says 30 minutes of idling))

[realtimclemans]

"30 minutes of non-interactive use before project stops" but within minutes of going to another browser tab and coming back I find that my project has stopped running.

[williamstein]

Projects normally run for 30 minutes before being shutdown. They very rarely might shutdown earlier if the node they are running on is terminated for some reason, and we can't guarantee that won't happen without charging 10 times as much...

The solution to this issue should be to add a link to where it says "30 minutes of non-interactive use before project stops" to the docs and in there say "we do not guarantee projects will always run for at least 30 minutes; if you need guaranteed uptime, consider running cocalc-docker on your own infrastructure."