Build svn releases with documentation from a matching branch

This is a quick hack to get the last 2.4 gnucash build
use the 2.4 branch of the documentation.
This should be implemented properly for git as well still,
but may need some more thought.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23543 57a11ea4-9604-0410-9ed3-97b8803252fd