Add doc(cfg) to read_from_io and write_to_io

[google-labs-jules]

Added #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))] to FromBytes::read_from_io and IntoBytes::write_to_io in src/lib.rs to ensure feature requirements are visible in the documentation.

Closes #2059

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PR created automatically by Jules for task 17925310465056035003 started by @joshlf

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.14%. Comparing base (7b1260f) to head (d0d0496).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2786   +/-   ##
=======================================
  Coverage   88.14%   88.14%           
=======================================
  Files          20       20           
  Lines        5450     5450           
=======================================
  Hits         4804     4804           
  Misses        646      646           

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