Pre-0.5.0 release: fix doc warnings and add missing crate-level docs

[dyreby]

Context

Preparing to publish 0.5.0 to crates.io. A doc review turned up a few things to clean up first.

Items

Broken doc links in twine-observers lib.rs

Lines 26–27 link to PlotObserver and ShowConfig via intra-doc links, but those items are behind the plot feature gate. When crates.io builds docs without that feature, rustdoc emits warnings and the links are dead.

Fix: use cfg_attr(doc, doc = ...) to conditionally qualify the links so docs.rs resolves them when the feature is enabled.

No crate-level doc on twine-core

crates/core/src/lib.rs has no //! comment. This is the front page on docs.rs — needs at least a short description of what the crate provides.

No module-level docs on equation and optimization in twine-solvers

crates/solvers/src/equation.rs and crates/solvers/src/optimization.rs have no //! module docs. These are navigational entry points on docs.rs.

[dyreby]

PR #265 also includes two README changes beyond the three doc items in the issue:

1. Clarified the GoodEnough paragraph — the original text ("GoodEnough is not tied to bisection. It works with any solver...") read like GoodEnough was a real framework type. Reworded to make clear it's an example illustrating cross-solver genericity via capability traits.

2. Fixed a broken import path — the observer example had use twine_observers::{HasResidual, CanStopEarly}, but those traits live in twine_observers::traits, not at the crate root. That import wouldn't compile. Considered re-exporting at root for ergonomics, but decided against it — if the crate grows, removing root re-exports would be breaking. The traits:: path is a small cost that keeps the API stable.