Add slow initiation section/warning to Downloads/Plotting.html

[cosama]

Importing Plots and the first call to the plot function usually takes a bit of time (20-30 seconds). This has been discussed in issue JuliaPlots/Plots.jl#917.

https://julialang.org/downloads/plotting.html is most likely the first resource most people new to Julia and interested in plotting will seek out. I think it would be important to add a section discussing this (or a warning) to that page and add some resources that help to adopt a more julia like workflow that reduces the impact of this. Maybe https://docs.julialang.org/en/v1/manual/workflow-tips/index.html could be a start, but I'm sure there are even better articles about that.

[ViralBShah]

Maybe submit a PR that sweet the right expectation on first use?

[ViralBShah]

There have been significant improvements on this front, and it is a topic of further ongoing improvements.