Improvements over actions available on the layer for QFieldCloud

[suricactus]

What I have realized is that we are missing a structural description how the whole thing is supposed to work. Main pain points:

• QGIS project is described multiple times in the docs
• layer actions are described multiple times in the project

Supercedes #452 , thanks @itsakifa

[suricactus]

@itsakifa I assume it is good enough.