"Billing Account" vs. a "Billing Project" in account setup

[avahoffman]

Note Katie's comment here. We want to make sure the text makes sense with what will happen later:

!!! This issue should also make sure there is an appropriate link in Class Setup back to Account Setup !!!

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I think it would be helpful to remind people what a "Billing Account" vs. a "Billing Project" is. This got a bit long, but here's a draft:

Reminder:

Google **Billing Accounts** are created directly on Google Cloud Platform and are used for organizing *funding sources* (e.g. credit cards, cloud credits).  It generally makes most sense to have one Google Billing Account for each source of funding.  Typically you will either have

- Course development and teaching funded by the same source, in which case you would use the same Google Billing Account
- Course development funded separately from teaching, in which case you would create a second Billing Account for your class

Terra **Billing Projects** are created through Terra, and allow you to associate your Terra activity with the correct Google Billing Account.  Google reports charges at the level of Billing Projects, so it is useful to create separate Billing Projects for each category of spending you would like to track.  Typically for a course you will want three Billing Projects (regardless of how they are funded):

- **Course development** for your activities as you prepare for the course
- **Instructors** for instructors as they teach the course
- **Students** for students as they participate in the course

These Terra Billing Projects may all point to the same Google Billing Account (if everything is funded by the same source), or may point to different Billing Accounts (if e.g. course development is funded separately from students' computing)

Originally posted by @KatherineCox in #69 (comment)