This attempts a complete revamp of the memoization tutorial. Since a lot of the boilerplate around using memoization is gone now, the old format doesn't make as much sense.
I'm thinking it actually makes more sense to have a memoization overview page than a tutorial at this point. I tried to go in that direction by using more self-contained code examples, and revamping into smaller, more targeted sections that people can search for specific information in.
In addition to general cleanup, I think the biggest question for me is how to show people memoization "working". In some sense, it might not even be necessary to do this, but it would probably be useful for initial sanity checking purposes.
Another note: in this current iteration, I have removed mention of the `list_versions` CLI. Given that we have relatively low CLI traffic, it seemed mis-targeted. I'm open to other opinions around that though cc @sandyryza .