Document project maintenance module

[nonprofittechy]

SuffolkLITLab/docassemble-AssemblyLine#815

[nonprofittechy]

Note that this has been transferred to the ALDashboard; it's still a commandline tool and ALDashboard isn't part of the auto-documentation that we have in this repo.

[nonprofittechy]

In #564 we added ALDashboard to autodocumentation, so I think we just need a better docstring that explains how to run the project_maintence.py file.