Standardize on 'Expenses" in account names

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.

Initial documentation for Income accounts, including tagging requirements.

Implemented in the config-accounts.ledger is an incomeChecker() function that
matches the documentation requirements.

Require and Entity and TaxImplication tag for outgoing payments over $10.

This now enforces what was said in the documentation in the previous commit:

The `TaxImplication:` tag is used for all `Asset:` accounts when the
transaction includes a payment of $10.00 or more leaving the
account. .... An [`Entity:` tag](entity-tag) should always go along with a
TaxImplication tag.

Initial documentation of the Program: tag.
Includes introduction of the section about making use of these tags.

Split out config-npo.ledger into four different files.

I realized that it makes more sense, even if it does create extra files, for
the accounts, commodities and tags declarations of the project to be
carefully split into different files. It's definitely more didactic to have
separate files for these declarations, to note that they are, in fact,
separable.

More to that point, users who cut-and-paste from this project into their own
setup will likely be least interested in the chart of accounts, which is
likely to be the longest file by far. The tags will probably be the most
interesting, so it's important they are in a stand-alone file that can be
easily found.