Initiate sequence diagrams

Context

This MR adds the first sequence diagrams.

It covers two usecases:

• Import Puzzle
• Solve Grid

This aims to be sufficiently structured to be taken as an example for the other usecases.

Like the rest of the documentation, the diagrams are written using PlantUML.

See also #26.

What has changed?

Main Changes

doc

Added sequence diagrams about the Import Puzzle and Solve Grid usecases.

Side Effects

doc

Created dedicated PlantUML files for component diagrams, just as for sequence diagrams: This allows to display the diagrams on IDE or platforms not supporting PlantUML code block rendering (e.g. GitHub), as well as to correctly display embedded images.