Fix long doc build time for single figure in intro to Catalyst

[TorkelE]

This figure in the introduction to the docs:

still causes really long doc build times.

Changing the code block to

plot(sol)
plot(sol, density = 10000, fmt = :png) # hide
nothing # hide

reduced doc build time from 700-800 seconds to around 150 (after the first run which compiles stuff). Just reducing the density did not necessarily seem to have that effect though.

Does not need to be fixed soon, and I am not sure what the best approach is, but would be nice to figure out at some point since it makes writing documentation a lot more annoying.

[isaacsas]

This seems like a documenter bug — it seems to be ignoring the request in the doc to plot this as a png and is still plotting it as a svg.

[isaacsas]

We shouldn’t reduce the density — it leads to a misleading plot that does not correctly illustrate the noise in the model.

[TorkelE]

I checked the plots with/without density and looked fine, but if you saw a difference then Ok.

[TorkelE]

The Documenter people suggest that the solution my be to use DisplayAs.jl, and that might work better than simply designating :png in the plot command. https://tkf.github.io/DisplayAs.jl/dev/#DisplayAs.PNG

[isaacsas]

Closed #1006