Document commands used to initialize the app

[danpastori]

Issue description

We have a few commands (and will have more) used to initialize the application. These should be documented.

What is expected?

I should easily be able to initialize the application using the commands found in the documentation.

Document the following commands:

• ff:seed-categories
• ff:set-up-institutions
• ff:initialize-demo
• ff:initialize-fresh