Ensure each core package's README lists purpose and accurate usage examples

Most of the packages in the `core` repo have a very spartan README. While this makes it easy to maintain, it is not very helpful for contributors.

Ideally, each package's README should describe its purpose and provide usage examples.

As a bonus, we should consider finding a way to make the usage examples executable, and run them in CI (similar to [Rust's documentation tests](https://doc.rust-lang.org/rustdoc/documentation-tests.html)).

Start by surveying the package READMEs in the core repository and identify which packages lack a clear purpose or accurate usage examples. Update the relevant READMEs, then verify that each describes its purpose and shows usable examples; the optional executable-example and CI work is a separate, broader follow-up.

Written by the indexing model from the issue text.