Build docs.rs documentation with `doc_auto_cfg`

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When building documentation for docs.rs, use the doc_auto_cfg feature to highlight which crate features are required for a given API. The feature is not used when building docs locally, as it requires the nightly toolchain (which docs.rs already uses).

You can check how the new docs will look by running:

RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features --open