Replace the Tutorial with the Guide.

The Guide isn't 100% perfect, but it's basically complete. It's
certainly better than the tutorial is. Time to start pointing more
people its way.

I also just made it consistent to call all things 'guides' rather than
tutorials.

Fixes #9874. This is the big one.

And two bugs that just go away.

Fixes #14503.
Fixes #15009.