docs: split 'amhello Explained' node. * doc/automake.texi (amhello Explained): Split node ... (amhello's configure.ac Setup Explained) (amhello's Makefile.am Setup Explained) : ... into these two. (Top, Hello World): Adjust, and add @anchor for stable URL links. Suggestion by Karl Berry in automake bug#7766. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>