Geometry types - add info about Multi Geometries / adding new layers to the project

[alex-cit]

While adding info about mapping multipart geometries, I have noticed that we actually do not mention anywhere the types of geometry (MultiPoint, Point, MultiLine, Line, MultiPolygon, Polygon). Also, the only place where we describe how to add new layers to the project is the tutorial Creating a Project in QGIS- I think it would be worthy to also include it in the QGIS Project preparation.