Docs: add #[doc(cfg(feature = "...')) to feature-gated structs and trait impls

[FreezyLemon]

This will add a note "Available on crate feature <...> only." to feature-gated structs, impls etc.

One random example of another crate that uses this is tokio. If you want to try it locally, you can run

cargo +nightly rustdoc --open -- --cfg docsrs

and it should show that some distributions need nalgebra, and that the trait implementations for rand::Distribution need the rand feature.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.94%. Comparing base (c866d42) to head (656f1c7).
Report is 13 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #293   +/-   ##
=======================================
  Coverage   93.94%   93.94%           
=======================================
  Files          52       52           
  Lines       11783    11783           
=======================================
  Hits        11070    11070           
  Misses        713      713           

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[YeungOnion]

Thanks for this, was wondering if there was a way to not manually include it as a note in the docstring.