Match CI doc testing with docs.rs.

[xStrom]

• Remove -Zunstable-options -Zrustdoc-scrape-examples from CI. It wasn't even enabled for docs.rs and we don't have any examples, so not worth enabling scraping at this point.
• Treat doc warnings as errors. Historically we've not done so due to various rustdoc bugs giving false positives but I got it to pass without failure right now, so perhaps better times have arrived.
• Pass --all-features at docs.rs to match our CI and reduce the maintenance burden of manually syncing the features list.
• Enable the doc_auto_cfg feature for docs.rs which will show a little tip next to feature gated functionality informing of the crate feature flag.