Add tutorial set-up code

[TorkelE]

Should solve #1105

Right now I just add a simple example for the CRN library example. If we are happy with text and approach we can copy across all (or some?) of the doc pages.

This is how it looks like by default:

And this when you click to expand the menu

[TorkelE]

Is technically everything at tutorial though? Maybe we write "Environment set-up" and keep it at that, or something else. Or "Julia environment set-up".

[isaacsas]

Maybe say "Environment setup and package installation"?

[TorkelE]

That sound good

[isaacsas]

Maybe say "Environment setup and package installation"?

[isaacsas]

It might be nice if this were a (Julia?) function we could call dynamically with a list of packages and it automatically generates this block (and maybe it should also autogen a block that loads the packages with some boilerplate text but isn't hidden)? Then in all the docs we could just call build_header(vec_of_pkg_names)? That would standardize the optional construction of environments, package installation, and package loading.

[TorkelE]

It should be relatively straightforward to write something like that.

Not sure about gathering all the using ... statements in the beginning. I f eel in many cases it is nice to have them appear as they become relevant (especially in doc pages covering several stuff, and where some relevant package is just relevant to a small subsection)