ci: build docs the way docs.rs does

[zoedberg]

cargo docs-rs reads [package.metadata.docs.rs] and runs rustdoc with the same flags docs.rs uses, so a metadata mistake fails in CI instead of on docs.rs after publishing. The plain cargo doc step is kept for -D warnings, which docs.rs caps to warn.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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