docs: replace concepts doc

https://opensource.axo.dev/cargo-dist/book/reference/concepts.html

This is super out of date and confusing because it's very internal. I should rewrite/delete it, and/or make it clear it's developer docs.

Start with the concepts documentation at https://opensource.axo.dev/cargo-dist/book/reference/concepts.html and compare its content with the current project. Decide whether it should be rewritten, deleted, or clearly labeled as developer documentation; done means it is no longer out of date or confusing about its audience.

Written by the indexing model from the issue text.