Add instructions for SpineOpt-plugin setup in Toolbox

[nelliputkonen]

There is a gap in the installation instructions between SpineOpt installation and Setting up a workflow: Adding the SpineOpt-plugin to Toolbox view, please add.

Other comments from installation:

• Add instructions to install Julia & SpineOpt via Toolbox Settings
• Add to instructions that Toolbox automatically finds julia.exe from PATH location, and you only need to edit if it is not found (it can be confusing that the path is greyed out while in the example image it is black and editable)

[clizbe]

@jkiviluo Did you say cleaning up the installation instructions is something SpineToolbox is working on?