Finish "core data structures" documentation, review internal docs

[Blaisorblade]

@smarter notes in #3405 that

There's also a bunch of weird/outdated things in https://github.com/lampepfl/dotty/tree/master/docs/docs/internals that should just be removed since they just add confusion.

Core data structures qualifies; git blame points to @odersky in 2016. I'd prefer to read the updated version. If there's no chance, let's maybe drop it soon?

[odersky]

I think it would be excellent if someone else went through this and updated the description. I have already way too much on my plate. I believe it would be very instructive to compile a record of what has changed since then and summarize that in a writeup.

[odersky]

I had a look at it, I think we should just drop core-data-structures. There's nothing that's still valuable to know today.

[Blaisorblade]

Will do!