Docs: Documentation for negative balance work

Docs: Documentation for negative balance work

Documentation for negative balance settings in 2.9. This updates in this branch
include:

* Updates of the circulation, patron record, and library settings sections
to reflect the new features contained in the negative balance work.
* General updates, consolidation, and re-organization of some components
of the billing section.
* A new section on configuring and understanding billing in Evergreen.

Signed-off-by: Kathy Lussier <klussier@masslnc.org>