Unless a new toolchain update has been merged, users should not need to
rebuild their toolchain. Yet, the first thing they see in the build
documentation is to run `Meta/serenity.sh rebuild-toolchain`, which
might incorrectly lead them to use it whenever they encounter an error.
This is a waste of time and causes frustration.
Move any mentions of this option to `Troubleshooting.md` and add a note
to `BuildInstructions.md` about the toolchain build being a one-time
endeavor.
