Updates for Documenter v0.20.

[fredrikekre]

Requires JuliaLang/METADATA.jl#19158 and JuliaLang/METADATA.jl#18901 first.

[codecov-io]

Codecov Report

Merging #15 into master will decrease coverage by 22.05%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #15       +/-   ##
========================================
- Coverage   97.05%   75%   -22.06%     
========================================
  Files           1     1               
  Lines          34    44       +10     
========================================
  Hits           33    33               
- Misses          1    11       +10

Impacted Files	Coverage Δ
src/Remark.jl	75% <100%> (-22.06%)	⬇️

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