doc: reformat net-stack-arch doc for PDF output

While the two-column layout looked OK in the HTML output, it totally
broke the PDF version of the documentation.  So, this change puts the
picture before the text instead of using embedded raw html tags to
create a two-column look. (And fixes the PDF generated output.)

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>