Plots.jl broken on Github CI

[johnomotani]

On the last couple of PRs, precompiling Plots.jl has been causing segfaults. On #306 disabling julia-actions/cache in the documentation workflow seemed to avoid the problem, but on #308 all the jobs that try to import Plots are failing (the two documentation jobs, and test_scripts). The segfault seems to be related to linking the Python library .so file.

My favourite solution to this would be to remove Plots everywhere and use Makie instead. Makie is pure-Julia, so we would avoid any Python related issues.
I'm not volunteering to do the work for that though, so for now I've disabled plots_post_processing in the docs (so at least the rest of the docs get updated), and we can ignore the test_script failures for a while and hope that some package updates fix things soon.