Update Getting started section in User guide

[soininen]

• Take new screenshots
• Fix links to SpineOpt tutorials (Two hydro plants and Case study A5 lead to 404)
• The Toolbar now has many names: Generic items, Specifications, Execute and different Plugins. The specific names should be used to guide user to click in correct places
• Mention that specifications can have multiple source files
• Give the example specification a different name than the main source file - avoids some confusion
• The section goes into too much detail when it discusses the specification JSON. This is mostly irrelevant for a novice users.
• Update instruction on how to add a new specification from file