Add app instructions as documentation

[tamslo]

As a first step in the direction of a proper app tutorial (see #259), create app instructions as part of the documentation, with screenshots and text descriptions. The flow should resemble the envisioned tutorial flow (also see #259).

• Add overview of newest screenshots (e.g., table with numbered rows and description, to make clear where transitions are)
• Add instructions (subset of table with better descriptions)
• Point to the documentation in Add app tutorial #259 for content and replacement