MassInstallAction: Install the Documenter workflow on this repository

[timholy]

This pull request sets up the Documenter workflow on this repository.

[codecov-commenter]

Codecov Report

Merging #8 (a63d125) into master (5101876) will decrease coverage by 12.69%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master       #8       +/-   ##
===========================================
- Coverage   93.63%   80.93%   -12.70%     
===========================================
  Files           6        6               
  Lines        1005     1002        -3     
===========================================
- Hits          941      811      -130     
- Misses         64      191      +127     

Impacted Files	Coverage Δ
src/precompile.jl	0.00% <0.00%> (-99.16%)	⬇️
src/GtkObservables.jl	81.81% <0.00%> (-10.91%)	⬇️
src/extrawidgets.jl	94.26% <0.00%> (-0.64%)	⬇️
src/widgets.jl	89.21% <0.00%> (-0.62%)	⬇️
src/graphics_interaction.jl	94.60% <0.00%> (-0.05%)	⬇️

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