Bump numr to 0.8 and splintr to 0.21, raise the minimum Rust version to
1.89, and exclude tests and CI files from the published package. Build
docs.rs with the CPU-backed model gates enabled.

CI builds docs with warnings denied and adds a job that compiles and
lints every model gate on Linux and Windows. Rewrite the changelog and
README for the 0.3.0 surface and add third-party notices.