Switch to doc_auto_cfg

[pitdicker]

Since end 2021 docs.rs can make use of the doc_auto_cfg.
With it we can remove all the #[cfg_attr(docsrs, doc(cfg(feature = "...")))] lines.

Like the doc_cfg feature we used until now it is only available on nightly.

I also switched the arbitrary and unstable-locales features to visible on docs.rs.

Can be tested locally with:

RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --features=serde,rkyv,unstable-locales,arbitrary

[codecov]

Codecov Report

Merging #1305 (6e095e3) into 0.4.x (c4fc4c4) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            0.4.x    #1305      +/-   ##
==========================================
- Coverage   91.36%   91.35%   -0.01%     
==========================================
  Files          38       38              
  Lines       17042    17034       -8     
==========================================
- Hits        15570    15562       -8     
  Misses       1472     1472              

Files Changed	Coverage Δ
src/date.rs	35.86% <ø> (ø)
src/datetime/mod.rs	86.25% <ø> (ø)
src/datetime/rustc_serialize.rs	92.72% <ø> (ø)
src/datetime/serde.rs	87.56% <ø> (ø)
src/format/formatting.rs	94.75% <ø> (ø)
src/format/mod.rs	85.04% <ø> (ø)
src/format/strftime.rs	98.94% <ø> (ø)
src/lib.rs	95.58% <ø> (ø)
src/month.rs	75.29% <ø> (ø)
src/naive/date.rs	96.30% <ø> (ø)
... and 9 more

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[pitdicker]

I have removed RUSTFLAGS="--cfg docsrs" from the cargo-deadlinks lint, and made the lint check all features.
The cfg does not influence the links anyway, and can run on stable.