Checklist updates from the website revamp.

I've now made a new section about how to turn on pre-release mode,
because although it's been carefully made trivial within the website
repo itself, there are still several things in other places I need to
edit.

Also added a note to myself to turn off nightly pre-release builds
after the release has gone out. I found on a previous occasion that my
build machine had been building them for months after they were
needed, which was harmless to the website but a waste of CPU!