update Documenter version

[jverzani]

Sort of punted. Turned on warnonly for missing docs.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (6da2557) 75.64% compared to head (2350ff8) 75.64%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #539   +/-   ##
=======================================
  Coverage   75.64%   75.64%           
=======================================
  Files          38       38           
  Lines        3999     3999           
=======================================
  Hits         3025     3025           
  Misses        974      974           

Files	Coverage Δ
src/common.jl	80.15% <ø> (ø)
src/polynomials/ngcd.jl	83.20% <ø> (ø)
...c/polynomials/standard-basis/laurent-polynomial.jl	93.97% <ø> (ø)
src/polynomials/standard-basis/standard-basis.jl	70.84% <ø> (ø)
src/rational-functions/fit.jl	69.62% <ø> (ø)
src/rational-functions/rational-function.jl	70.58% <ø> (ø)

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