How to generate assembly schematic file in Altium
Do this in PCB.


In the PCB editor, enable a new mechanical layer.
Make the part names visible, and change (or copy) them to the new mechanical layer.
Add any additional notes or graphics on this mechanical layer, as desired.


You can then define a new Outjob to produce the desired type of output, with only the layers that you want visible.  Output types include, Gerber, PDF, Hardcopy...