An example of an invoice for a conference where the income type is split
between RBI and Donations. Include a ledger reporting example that
shows the total of donations, and explain in the documentation why
that's useful.
We've concluded that the top-of-hierarchy account name for Expenses
should be Expenses, not Expense. This change normalizes the tutorial
text and examples to Expenses.
Since this system relies so heavily on tagging, even though we assume the
reader is generally familiar with Ledger CLI, it's worth ensuring they know
the basics of how tagging works.
Also added herein is the example used in the text over to the Ledger file
itself.
