staging/hashmap: Fix outdated info

Cut out a lot of BS in the markdown to push the examples and functionality. Also fixed a bunch of outdated information.

I thought about changing `alt-key-types.rs` to demonstrate that you can index a `HashMap<String, _>` with `&str` but I didn't want to get rid of the current example which, though contrived, does demonstrate how to have a `HashMap` keyed by a custom type.