Add budget documentation

GnuCash help:
* 9.3.2: Clarify that the 'default budget' option is for budget reports
* Add a section about budgets to Chapter 4.
* Added link to "book options" in case users don't know what that is

GnuCash guide:
* 15.3: Describe budgeted income statement and budgeted balance sheet
reports from a conceptual perspective. Note: This doesn't necessarily
document the way these reports work in GnuCash.

Author: Robert Ratliff <ratliff.bobby@gmail.com>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash-docs/trunk@23339 57a11ea4-9604-0410-9ed3-97b8803252fd