Enable running multiple notebooks in parallel

[fonsp]

This was not enabled initially because Pkg is not thread-safe, and many notebooks included a "Pkg cell" that sets up the environment. Running mulitple notebooks in parallel was known to crash Pkg sometimes.

Nowadays, Pluto has a built-in package manager, and this built-in package manager is thread-safe (Pluto has a global Pkg lock). This means that we can safely run multiple notebooks in parallel (if they all use the built-in package manager).

We should add this feature, with a configuration Export_max_parallel_notebooks (default to 4).

[fonsp]

I implemented this in the secret WIP PlutoPages.jl... more info later!

[theogf]

Is there any update on this :) ?