Using DemoCards.jl for plot examples gallery

[sethaxen]

Both Plots and AlgebraOfGraphics use DemoCards to build and display deliver galleries of plots in a Documenter-compatible way. Each example gets its own file written as a Julia script with a Literate.jl/Documenter.jl hybrid syntax.

There would be some upfront cost splitting our large examples file into 37 individual files with these headers, but it would give a much better preview of our plotting functions.