feat(ci): deny cargo doc warnings

[LesnyRumcajs]

• added cargo doc checks to the CI with fail on warnings,
• fixed warnings.

Closes #2191

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.50%. Comparing base (9273717) to head (2b3bc2e).
⚠️ Report is 37 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2192   +/-   ##
=======================================
  Coverage   77.50%   77.50%           
=======================================
  Files         147      147           
  Lines       15743    15743           
=======================================
  Hits        12201    12201           
  Misses       3542     3542           

Files with missing lines	Coverage Δ
fvm/src/gas/timer.rs	87.03% <ø> (ø)
fvm/src/syscalls/error.rs	52.94% <ø> (ø)
ipld/amt/src/lib.rs	94.11% <ø> (ø)
ipld/bitfield/src/rleplus/mod.rs	90.60% <ø> (ø)
sdk/src/message.rs	0.00% <ø> (ø)
sdk/src/sys/mod.rs	0.00% <ø> (ø)
shared/src/sys/out.rs	0.00% <ø> (ø)

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