Update docs build

[t-bltg]

Common CI docs build sequence between PlotDocs.jl and Plots.jl.

Complements JuliaPlots/PlotDocs.jl#285.

[codecov]

Codecov Report

Merging #4175 (0f96ef4) into master (99ad317) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4175      +/-   ##
==========================================
- Coverage   77.60%   77.47%   -0.13%     
==========================================
  Files          28       28              
  Lines        7054     7054              
==========================================
- Hits         5474     5465       -9     
- Misses       1580     1589       +9     

Impacted Files	Coverage Δ
src/Plots.jl	78.26% <0.00%> (-2.18%)	⬇️
src/layouts.jl	75.51% <0.00%> (-0.89%)	⬇️
src/backends/gr.jl	88.77% <0.00%> (-0.33%)	⬇️
src/recipes.jl	63.56% <0.00%> (-0.12%)	⬇️

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