Docs: replace internals article

[NickCrews]

agree on a concepts article -- perhaps replacing the current one on internals: https://ibis-project.org/concepts/internals

Originally posted by @lostmygithubaccount in https://github.com/ibis-project/ibis/issues/8484#issuecomment-1985841689

I agree. This article can't decide if it is targeted for a reader who is a user, or a contributor. We should split it in two, one for each audience.

[NickCrews]

The user facing article I think should be a basic overview of how ibis works, but only to the depth needed to use ibis:

• expression building
• rewrites
• compilation
• execution