Add areaplot example to gallery

[briochemc]

Not sure this is the right way to add example to the gallery but hopefully this is helpful :)

[t-bltg]

@BeastyBlacksmith (or @johnnychen94), what is the criterion for orienting contributions to the new gallery (old examples) or the user gallery ?

[johnnychen94]

Here's my understanding:

• For examples that can be fit into the docstring, it goes here (the old examples); these examples have no differences from the examples listed in ?startswith and it simply indicates that these functionalities work or don't work for certain backends.
• The new gallery is more of a "how to make a beautiful plot result" collection; it doesn't necessarily work for every backend.

For this specific PR, I think it should be here.

[briochemc]

FWIW, I submitted this because someone on StackOverflow "looked at all of the examples in the documentation and could not find a similar graph" (similar to an areaplot).

[t-bltg]

Good point for the docstring. We could say that the examples working with all major backends GR, Plotly, PyPlot, PGFPlotsX are allowed to go into the regular Gallery.

So @briochemc for this PR to go in you'd need to:

1. stick to the format (run using JuliaFormatter; format(["src", "test"]))
2. generate the new reference images for gr by running the test suite, and submitting a pr to https://github.com/JuliaPlots/PlotReferenceImages.jl (see also https://docs.juliaplots.org/stable/contributing/#VisualRegressionTests).

[codecov]

Codecov Report

Merging #4178 (35247d4) into master (4f9a708) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4178      +/-   ##
==========================================
+ Coverage   77.55%   77.64%   +0.08%     
==========================================
  Files          28       28              
  Lines        7054     7054              
==========================================
+ Hits         5471     5477       +6     
+ Misses       1583     1577       -6     

Impacted Files	Coverage Δ
src/examples.jl	96.77% <ø> (ø)
src/axes.jl	85.26% <0.00%> (-0.23%)	⬇️
src/backends/gr.jl	89.01% <0.00%> (-0.09%)	⬇️
src/args.jl	74.39% <0.00%> (+0.15%)	⬆️
src/recipes.jl	64.23% <0.00%> (+0.77%)	⬆️

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