doc: If I patch 50 times between users opening app, how much do I pay

[eseidel]

Came up in a customer conversation this morning. Customer was concerned they were going to end up paying per-commit if they ended up wiring up shorebird to their hourly CI, etc.

In their case, customers only open their app about once a week, far less often than they commit to the app.

We could either include this as an FAQ on /pricing, or maybe as part of a calculator, e.g.

Calculator:
Approx. Monthly Active Users
Planned patch cadence
Approx. User Engagements Per Day