doc: Replace doc_auto_cfg with doc_cfg

[RasmusRendal]

In the newest unstable, doc_cfg replaces doc_auto_cfg. So therefore,
our docs no longer build on unstable, until we replace this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.89%. Comparing base (f7e2f1b) to head (42cbbc1).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #36   +/-   ##
=======================================
  Coverage   67.89%   67.89%           
=======================================
  Files           6        6           
  Lines         380      380           
=======================================
  Hits          258      258           
  Misses        122      122           

Files with missing lines	Coverage Δ
src/lib.rs	64.55% <ø> (ø)

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