Document how to use the financial summary screen

[halfbro]

There should be documentation for how the financial summary screen works, and how to open it.

There should also be info on how to take out a loan, how to start marketing campaigns, and how to set research funding.

[Gymnasiast]

ab42ad5 covers the basics of managing money in general. It mentions how to open the window and what each tab does, but could do with a bit more information about how loans and marketing works.