Docs: Documentation for negative balance work

Docs: Documentation for negative balance work

Documentation for negative balance settings in 2.9. The 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 understanding and confiuring billing section is still to come, but this
branch brings existing documentaiton up to date.

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