Fix some minor weirdness when Pilcrow is (unwisely) used as a library.

Pilcrow isn't meant to be used as a library, and the only public interface the `pilcrow` lib crate exposes is the CLI entry point. However, we will likely be publishing Pilcrow via crates.io (among other options) one day, and so it will _be usable_ as a library if someone's desperate enough to try. To that end, let's try to be good citizens.

This change fixes two issues:

* The docs contained links to internal items, which are not actually included in the library documentation. The links are simply removed; the uses of those items were already private anyways.

* The CLI `Error` type is no longer part of the public interface, using `impl Trait` (`impl std::error::Error`) shenanigans to hide the error type from callers. (To be clear, this would be _extremely_ rude in code intended for library use.) This frees us up to change the structure of the error type - or to replace it entirely - without making the world's most pedantic semver change in the process.

Merges lib-crate-weirdness into main.

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