fix: migrate doc_auto_cfg -> doc_cfg

[TroyKomodo]

after rust-lang/rust#138907 merged the doc_auto_cfg feature has been merged into the doc_cfg feature so our docs.rs builds started failing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.47%. Comparing base (6a11ba0) to head (273919d).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #4   +/-   ##
=======================================
  Coverage   45.47%   45.47%           
=======================================
  Files         102      102           
  Lines       10284    10284           
=======================================
  Hits         4677     4677           
  Misses       5607     5607           

Files with missing lines	Coverage Δ
embed-changelog/src/lib.rs	0.00% <ø> (ø)
postcompile/src/lib.rs	59.77% <ø> (ø)
tinc-build/src/lib.rs	1.63% <ø> (ø)
tinc-proc-macro/src/lib.rs	100.00% <ø> (ø)
tinc/src/lib.rs	100.00% <ø> (ø)